So you're driving up from KW?
Robbie's Marina at mm 77 in Islamorada would be fun for the kids. There's food, trinket shopping, and you can feed the tarpon.
Morada Bay restaurant in Islamorada was excellent. Great linguini carbonara.
MM 50 has a great Cuban sandwich and juice place, Juice Paradise. Good cafe con leche.
Drive around very slowly at Big Pine Key to see the key deer. There is a bar called No Name Pub that is totally covered inside by dollar bills.
Can't think of much else, since we didn't have any kids with us
Gawd, I loved my Keys vacation!! Have fun!