There are many threads here on Mont Tremblant; a search will turn up a plethora of good infromation. Here's a few tidbits for ya:
La Source in the center of the village is an indoor swim club that is a little kids paradise. Warning, once you take the kids there you will be going back every day.
Just wandering around the town window shopping.
There is a small skating pond.
There is an activity center desk in the village where you can book dog sled rides, snowmobile rentals, etc.
Maple syrup poured onto snow then rolled up on a stick for a nice sticky kid treat.
Plenty of top to bottom green runs.
Warnings: Very cold in mid-winter.
Very crowded mid-march when Ontario schools are on vacation. (not sure how it is when Quebec schools are out.
If you go when crowded make dinner reservations as many places will have hour-long waits.
Either get on the slopes early or late as there are long lines at the base lifts mid morning that usually become much shorter after everyone gets up the hill.
Great place to take the kids!