You gotta be careful at double branch on an outgoing tide when you're trying to come back in, especially with a 12 year old. The tide RIPS through there, also make sure you have a GPS it is extremely easy to get lost in the mangroves. An opening that you just went through will disappear against the background of other trees 100ft away.
Double Branch has some awesome scenery and it's really quiet as it's too shallow for any boats to get back there, it's in Oldsmar actually not too far from FAOIS and Coral Corral. This is probably one of the more secluded areas, if you go on a weekday there's a chance you won't see another person all day, even on the weekend you won't see too many people.
In Pinellas your best bet is Ft. Desto it's very open so no chance of getting lost, but won't be as fun as going through some mangrove tunnels or forest. This is a good start cause you'll usually be in less than 5 feet of water and there's lots of good things to see in the flats. This is probably going to be one of the busier places, if you're not fishing it shouldn't matter though.
Then in Tampa there's Weedon Island this has some of the best mangrove tunnels, there's one that's only about 3 feet wide and the mangroves close right above you. The little tunnels are blast as you feel like you're about to enter some lost paradise when you get to the other side. I always launched from Gandy and made the 2.5-3 mile paddle over , but there is a launch in the park somewhere. The no see-ums are brutal here in the Summer so bring bug spray.
Always make sure to check the forecast before going out, the wind is the biggest pain and always told me where I was going to go. It can also change direction out of nowhere and make the paddle back a nightmare. Anything more than 15mph just stay in, you have to paddle hard against the wind and cannot stop cause it'll just blow you back and fast. You can usually stick close to mangroves to stay out of the wind, which is where Double Branch came in handy.