Walpole and Surrounds
When asked about holiday down south, most people would think of Margaret River and the surrounding areas. However there are actually many towns down south that are equally attractive, with plenty of national parks, natural attractions, and cheaper accommodations.
One such town is Walpole, a cute little town surrounded by inlets, rugged coastline, and forests of exceptionally tall trees, some of which reach higher than 40m above the ground. To reach this place from Perth, travel south on Kwinana Freeway towards Bunbury and just outside Bunbury, turn left where it says "South Western Highway" towards Manjimup. The township of Walpole is about 120km further south from Manjimup, making it a total distance of 350km from Perth.

Mt. Frankland National Park
With such a distance, it is not a comfortable day trip for most families, so overnight stay might be in order and
Walpole Nornalup Visitor Centre thankfully has an online booking facility which makes it convenient to choose and reserve your bed. If you're the bush camping type,
DEC Campgrounds has a few sites near here you might want to check out.
Now that the accommodations are sorted out, next on the list is "what to do while we're there." It is not very hard to answer and below I have a few locations which are popular and for good reasons too.
Valley of the Giants
Needless to say,
Valley of the Giants is the most popular tourist attraction in the Walpole area. In fact, it is so famous that many tour companies conduct day tours just to visit this place, leaving Perth early in the morning and returning early evening. I would not recommend such a day trip, however, since it would not allow opportunities to visit other attractions in the area. Well, unless you're a tourist with only one day to spare, what can I say?
The main item in the menu here is the Tree Top Walk (entrance fee applies). It's a bridge-like structure allowing you to take a walk high up amongst the foliage of the tingle trees. It's not as scary as it looks, the structure is quite stable and if you don't like heights, just refrain from looking straight down and you'll be fine.

Tree Top Walk
Valley of the Giants is more than just Tree Top Walk though, so don't be discouraged if your budget doesn't include the cost of entry tickets. Come anyway and visit the Ancient Empire Walk, a pleasant meandering walk through the floor of the forest. Down here you won't get butterflies in your stomach, but you can meet Grandma Tingle, a tingle tree with a granny face on one side (a stretch of imagination required here), you can walk through trees without being a superman (the trees obligingly have cavities big enough for people to walk through), and you can learn about the plants by reading the information plaques provided along the path.
Wilderness Discovery Centre
And while you're there, make sure you stop by the Wilderness Discovery Centre. It has interactive displays, videos, stuffed animals, and a board replica of an old tingle tree with a cavity big enough for a small car to drive through. Sadly the real tree died from excessive human attention. Tingle trees have relatively shallow roots, so too many people trampling around would compact the soil too much for the roots to survive. That's why we need to keep to the designated paths in the area.
Coastal Attractions
Walpole is located quite close to the south coast and a couple of coastal attractions worth visiting are the Conspicuous Cliffs and Mandalay Beach.
Conspicuous Cliffs is not far from the Valley of the Giants. If you drive east from Walpole (towards Denmark), the turn-off to Conspicuous Cliffs would be on your right, about 150m from the Valley of the Giants' turn-off (which would be on your left).

Conspicuous Cliffs Lookout
The main attraction for me here is the lookouts. There are a couple of them, one higher than the other, and you'd need to go up a flight of wooden stairs to reach it. There is access to the beach, but I think the beach is not as beautiful as Mandalay Beach.
Mandalay Beach is located about 13km south west of Walpole. From Walpole, drive west (towards Manjimup) and just after you see the sign "Crystal Springs," turn left onto the gravel road. In spring time, the forest floor along this road is carpeted with yellow wildflowers, very pretty and sunshiny-looking.

Mandalay Beach
Once you reach the carpark, a short boardwalk would take you to the lookout. The beach looks amazing from here, with a wind-sculpted stretch of sand, rippled waves, and if you're lucky, the wreck of the Norwegian ship Mandalay visible above the surf. There is beach access via wooden stairs, but watch the little ones as the wind could be strong and the water rough.

Mandalay Beach's Lookout
Mandalay Beach is included in the
101 Best Beaches in Australia by Prof. Andrew Short and Brad Farmer. Not too many WA Beaches are included there, so it is kind of special.
Mt Frankland
Mt. Frankland National Park is located north of Walpole, and can be reached via North Walpole Road. It is perhaps not as spectacular as many other national parks in WA, but Mt Frankland has a good short hike suitable for all ages (except very young toddlers), and that's definitely a plus for a family holiday.
Despite its remote location deep in the forest with unsealed road access, the hike up Mt. Frankland is surprisingly well-appointed. The walk path is paved, and the difficult part is made immensely easier with sturdy metal ladders. Even preschoolers would be able to navigate the ladder with a little support from Mom or Dad (mostly psychological support to give them confidence).

Ladder to help you up Mt. Frankland
There is a 360 degree view from the top with greeneries upon greeneries whichever way you look. And the fence surrounding the bald top made me feel better about the sheer drops on all sides.

Top of Mt. Frankland
Giant Tingle Tree and Circular Pool
After Valley of the Giants, it may seem pointless to see another giant tree. However, if you have the time, why not visit this particular one together with Circular Pool as both are located on a one-way road circuit.
The turn-off to these two attractions is approximately 1 km east of town. Giant Tingle Tree is just that, a huge tingle tree, with a diameter bigger than the ones in the Valley of the Giants. It's so big because it's old, and this particular one is believed to be the oldest living eucalypt in the world.
Further along the one-way road, don't forget to stop by Circular Pool. This name derives from the river currents which flow in circles, creating patterns that are visible due to the foam floating on the surface. The foam is due to a type of eucalypt growing there, which stains the water and creates frothy foam that looks like cappuccino topping. If you wish, you could bring your lunch here as it has pleasant picnic tables overlooking the river.

Frothy Foam around Circular Pool
When you're done, just continue along the one-way road until you hit North Walpole Road. Turn left and in no time you'll be back on the highway, a little west of town.
Walpole Town
Last but not least, let's see what Walpole Town has to offer. In terms of facilities, it has an IGA, a couple of petrol stations, the Walpole-Nornalup Visitor Centre, all along the highway.
For swimming with children, it's probably best to visit Coalmine Beach, just south of town. It's an inlet beach, not very exciting in terms of waves, but is safe exactly because of that.

The calm water of Coalmine Beach
The Coalmine Beach is also part of a scenic drive called Knoll Drive. It goes around in a circular fashion passing close to the inlet in several places, so that you have the view of water as well as the trees surrounding it.
For such a small town, I'd say Walpole offers a big holiday for the whole family, with plenty of opportunities either for relaxed passivity or more active pursuits. Definitely an option to be considered for your next family trip.