In theory, you COULD spend $100 a day per person, without breaking a sweat (Okay, so maybe you would sweat a little if you were spending that much per day in extras)...but you could also get by on a lot less than that. You could, in theory, get by without spending hardly anything beyond the dining plan.
If you think your kids would hold on to them, you could buy each kid a small Mickey on the first day (or whichever animal they like), and let them carry it around each day, that way you wouldn't have to buy something new each day.
However, if you've never been there before, you'll most likely be tempted to get each kid something new each day (or at least an item from each park). There are plenty of items to be had in the $10-15 dollar range, so that is a fairly reasonable budget for kids as young as yours.
(Costumes will set you back about $50 plus, if you get all the accessories, and there are also plenty of items in the $20-25 range too. A "first haircut" complete with pixie dust, certificate and "first haircut" Mickey ears will run you $17 in the MK)