Both the Destin and Panama City Beach jetties have protected areas that are ideal for kid's snorkeling. There are plenty of fish and many crabs and hermit crabs that your kids will love to observe. Relatively large hermit crabs are easy to capture. The only problem is the only good time to go is at high water slack tide, when clear water from the Gulf comes in and currents are least. However, this time of year, and increasingly as you move into fall, high tides for a particular day often occur during the night. You will have to consult the tide tables for Panama City (St. Andrews Bay inlet) and East Pass Destin to see what days have favorable tides for snorkeling.
I have an extensive collection of underwater photos I have taken while snorkeling, mostly in Destin. Here they are:
Destin Florida snorkeling - a set on Flickr
Most of these were taken under good conditions; often the visibility isn't quite as good as these photos show. Under calm conditions at high tide in the summer at Destin, I'd estimate the visibility at the jetties is typically about 20 ft.
Here is an informative post I wrote on TripAdvisor several years ago about snorkeling in Destin:
Snorkeling in September? - Destin Forum - TripAdvisor
Cursor down to see the long entry by "Hawkfish" (my name on TripAdvisor).
If you rent a pontoon boat in PCB, it can also be fun to snorkel behind Shell Island in the grass flats. The visibility isn't that great, and there is not the diversity of fish that you will see at the jetties, but it is extremely easy with shallow water and no current. If you poke around long enough, you will usually come across some interesting things. Small stingrays can be common there.