Me and my family visited during December with four children. They were spread out in ages from 7-15, so that can help you think of the age group who would enjoy it.
When we arrived, it was already pretty busy in the late morning. There was an abundance of parking which was nice.
Most of the viewing center is outdoors, so keep that in mind if the weather isn't very warm like it was when we visited. When we were there, it was about 60 degrees or so outside.
There are a few piers for seeing the manatees just inside the viewing center. There were pretty large and the manatees weren't hard to find.
We saw dozens of manatees while we were there. Mostly we saw just lumps just underneath the surface of the water, but of course that is just the nature of the animals. We were able to take a lot of pictures and it was very fun.
Elsewhere in the viewing center, there is a educational center, gift shop, and a second pier.
The educational center has mostly reading about the manatees. Since our kids like to read, they enjoyed that area, but there are also some hands-on things in there as well.
The second pier is located straight back. It's a short walk to get there, and you also pass through a tunnel of mangroves. The tunnel of mangroves is a great place to take family photos and stuff like that.
In the back pier, you could see some manatees, but there weren't as many.
The gift shop was a little on the small side, but for most, I don't think it's a dealbreaker really.
All in all, this is a great place, especially since it's free. Our kids came to this place rather skeptical, but they ended up enjoying it. It's a very nice place to see manatees without having to pay.