They will love Magic Kingdom. Most of the other parks might be a bit much for them (not to mention will make you crazy).
Our son loves the character meals. We have gone every year for the last five years and have either done breakfast or lunch at either the Crystal Palace with Pooh and the 100 acre woods characters or a meal at Chef Mickey's. My son enjoyed both of them but we celebrated his bday at both so it's a bit different if you tell them it's a bday.
They do a lot of fun additional things if it's celebrating a bday.
Anyway my advice as a parent who has been to all the parks would be to start with Magic Kingdom and see how you do before venturing on to other parks. Your kids will be exhausted and so will you.
I am sure some other parents will have some more fun suggestions.
What side of town in Indy are you on? If it's north I can tell you some good kid friendly places.....