I hate to break this to you but all your food is not paid for. You still have to pay gratuity on your meals. If it's a group of 6+ people at the table service, a minimum 18% is added to the bill that you owe when done. Being on the meal plan your all going to be ordering the mega expensive everything so that's going to be a lot more than you expect. Just be ready for it so it's not a surprise.
You have the kids budget down and that's going to really help how much additional you need or will spend. I personally budget about $1000 for any vacation on "other costs". We never spend it, but we also never go home upset that we over spent after unforeseen expenses. Most of the time, we go home really happy on how much we didn't spend and what is left over is the start of the next trip budget. The "kids budget" would be in that number for me.
For us, it's generally splurging because we ran into some events, gifts, or weather required more beverages or clothing and we decided to just buy more things while there. For example, did I really need some frozen coffee drinks at starbucks and joffreys while in each park? sure it was hot but I could have went for free water at a quick service. Last trip, a month ago, we were also a little under the weather so we had some more down days and ended up doing things like AMC theater to see a movie and that expense. My daughter did get caught in a really quick but very wet rain. I went for the extra expense to get her dry clothes from a gift shop instead of being wet trying to get to a change of clothes at the resort and back.
I keep enough cash in my pocket to make me comfortable but we do most of our spending as room charges so we don't actually actually touch that the entire trip but it's nice to just have it.