For a short excursion that's great for toddlers+, visit Wolfe's Neck State Park in Freeport. They've got a couple miles of beautiful, mostly flat trails, some of which are stroller/wheelchair accessible. Take a picnic to the rocks along the Casco Bay trail, then explore the tide pools and watch the ospreys in their nest on Googins Island. We can easily kill a couple hours here. With more time, walk the other easy trails through the forest to the cliffs above the Harraseeket River. WNSP is also great for beginner-friendly snowshoeing in the winter.
Make a longer outing by visiting the nearby Wolfe's Neck Center for Agriculture and the Environment. It's a super kid-friendly working farm with loads of educational info and hands-on experiences. They also have wooded trails and ocean-front camping.
Note: The only time (so far) I've gotten brown tail moth rash was from Wolfe's Neck State Park. In some years the moths are really bad there, so check government web sites if you plan to go in late spring/early summer. If you do get the rash, I recommend you contact a compounding pharmacy for a special spray brew they make. It's genius.