John Pennekamp state park has lots to do for families - several beaches, a boat ramp, snorkeling, glass bottom boat, aquarium and walking trails. Definitely bring the bikes and beware that if you are tenting it the ground is mostly rock, so be sure to bring some strong L-shaped aluminum tent stakes. We had quite a time setting up the PahaQue tent but it was so worth it!
Also note that these are not very private sites and there are no fire pit rings in the sites. If you want to have a fire at night you must bring your own fire pit or otherwise you can light some wood in the raised grills at each site. Sundowners is a great restaurant in Key Largo but our favorite restaurants are in Islamorada which is about a 25 to 30 minute drive. We love Lorelei for the fun and low key atmosphere and Morada Bay as a nice dinner - we had my mom's birthday dinner there.