We usually rest the first day because traveling seems to wear the kids out, and we like to start fresh the first full day. Sometimes we have a character meal and then explore our resort.
However, if we had passes to use, MK is the place to go. Nothing says welcome to WDW like the MK. Because of your kids ages, I'd head straight through the castle (so magical) to Fantasyland for those fun rides (winnie the pooh, peter pans flight, Dumbo, etc).
Have a great time!!