"It is not a bottomless pit you know."
Children do find this concept hard to understand, naturally.
I can remember being a child myself and seeing my Dad write a cheque. No money appeared to change hands. "It's free money". I thought. So, when he said that he couldn't afford something, I, quite sensibly, (I believed) suggested that he write a cheque. Plenty of confusion ensued, and eventually it ended with my understanding, finally, how cheques worked.
Of course, we rarely use cheques nowadays. Instead we have a clever piece of plastic that goes into a funny machine. How could a child possibly be expected to understand that that is the same as using the hard earned pocket money from their piggy bank? The truth is that young children cannot, and so we have to find ways to entertain the young, that are actually free.
So here it is, part one of how to entertain the children with no extra money leaving the bank account, by cash, cheque, card or even barter.