A.M.
It's a bit of a drive, but Perpetual Motion in Lowell is really great for toddlers/pre-schoolers. It's like an outside playground, only inside -- swings, sandbox, climbing structures, giant legos, etc. Only kids 5 and under can play in the young area. On the other side of the space is an area for 5+, with climbing, a big foam pit etc. appropriate for older kids. It's about $7 a child for the "little kids" and you can bring your own food.