You can deposit money with Guest Services and have it 'on your account'. I think you'd have to do this with the Guest Services desk after you board and not at checkin.
They wouldn't know how much was left of their money. You could go to the Guest Services desk and ask for a printout of your charges whenever you wish, and it is broken down by KTTW card number, but you have to manually check it.
As far as I know, you can have charging privileges allowed or not allowed on the kids' cards, there's no dollar amount limit. But after you manually checked, once they reached their limit, you could have it turned off from that point on, if you wanted.
I think your plan is a good way to handle their money and it sounds appropriate. Only one problem that I can see now, and I'm predicting this could happen from my two DGDs. Suppose the oldest spends the $50 using the KTTW card, but the youngest only sees $30 worth of tempting items? Naturally, he would want his $20 back, since he didn't spend it. Suppose that you had spent $200 on regular things like Shore Excursions, the Spa, or beverages, so your account would be $100 deposited, $80 spent by the two kids, and $200 by the family for a negative balance of $180. Guest Services would probably be reluctant to give back a $20 bill when you owed $180. So you'd have to be prepared to give him back the $20 in cash whenever he was ready for it, just a minor problem.
I can see my two DGDs and one of them is ready to buy the first nice thing that she likes. The other one wants to see every item available and then go back and choose one.
hope that helps