Google "Destin for kids" and you will come up with a bunch of stuff. Just know that Topsail is a good 15 minute ride from Destin itself. There isn't anything for them to do right outside the park, except ride a bike towards Santa Rosa Beach.
Topsail itself has a pool, shuffleboard and some other things for kids to do. My 13 yr old daughter loves it there.
Brian
2013 Thor Chateau 31L