You've got them at the low maintenance stage there
What do you consider the high maintenance phase? I figure as they get older, then costs will be greater, but less frequent. I.e, we won't be paying for swimming lessons each week but we might have to splash out the equivalent of a few months lessons on a school trip.
When they're old and enough to be useful, then they'll get pocket money to buy some of their own stuff, but at least we'll get a few chores done in return!