K.C.
Lake Mary is a wonderful area! Lake Mary elementary is a great school and has been consistantly for many years. Most schools in Seminole county are pretty amazing! (My husband and I are both Seminole county teachers :)) I would also reccomend looking in Oviedo (tons of city wide family events all year round) and Winter Springs (wonderful family area, great parks, great schools, quite, etc.)
As far as workout centers there are planet fitnesses everywhere, Lifestyles in Winter Springs and Lake Mary, an Oviedo YMCA, I own Stroller Strides Central FL (www.strollerstrides.net/wintersprings) and for the kids in Lake Mary there is a My Gym and The Little Gym (which I bring the baby to and LOVE!)
There are great parks all around Seminole County and lots of new splash parks popping up all around though most of those close in October for the winter.
Casselberry also has some nice areas and there is a brand new place there called Amaya Papaya for your younger two to play. We went to the grand opening this weekend and it was sooo much fun. There are lots of great moms groups on meetup.com for central FL and there are always tons and tons of kids activites on there!
Hope this was helpful. My husband and I moved to Central FL bout 6 years ago and I can't imagine living anywhere else! Please let me know if you have any other questions that I can help you with!