Not exactly a fell walk, but my firstborn is now 3 and like you I've been somewhat restricted in recent times and am dying to get him up the hills. I also have a second who was born on Christmas Day, so like you again have another one to think about.
Anyway, my plan is to take no. 1 up Snowdon this summer via the 'Tourist route' from Llanberis. It's about 3 miles I seem to remember, and if he's a bit 'cream crackered', we can come down by train.
Summer 2007 we were driving past Ingleborough which I showed him (for once there was no cloud cover), so I asked him if he'd like to climb a mountain and he said yes, so I have to do this. Sure he doesn't understand fully, but the beauty of the Snowdon tourist route is that there are stations all the way up, so if things go wrong you can always bail out.
S.