Oooh I was just there last weekend. Callendar is nice for a mooch around the shops (fab chippie) and there's a great playpark and short walk for the kids.
Aberfoyle is nice too and there's small (very small) kiddie play bit in the visitor centre which my two played in for about an hour (I was v. v. bored towards the end)and a little playpark round the corner. Also, we went to the wollen mill shop which had some fab kids clothes and a wild bird display outside. Watch out for deer on the roads though!
I third Blair Drummond, a great day out if the weather is nice, you could spend an entire day there, or nip into Stirling for a look round the castle and shops in the city centre.
Falkirk Wheel is good too, with another good kid's playpark at Callendar park and leisure pools in Camelon and Grangemouth which also has another playpark. (Can you tell I know this part of the country very well, grew up there!?).
As you're in Central Scotland, if you've got a car or feel like jumping on a train, I would also go into Edinburgh for the day - go to the zoo, Chambers Street Museum, Museum of Childhood, Castle etc.
We also went to some of the beaches, which are lovely, but mibbe a bit cold at this time of year.
Hope this helps,
SB