My son loved paging through the MM when he was 3, and still does now that he's 4. My daughter, on the other hand, no longer finds it interesting at 7. My wife was worried, as she's very into Barbie and girly things...but then she asked to have a TMNT set with nun-chuks. I mean, when given the choice between Starfire and Barbie, Starfire wins every time. So it's all good.
Someone - I think it was Sandy from RPGnet - once suggested photocopying pictures of monsters in the MM and other gaming books and stapling the pages together to make a monstrous coloring book to keep kids occupied while you're gaming. Just a neat tip I wanted to pass along.
My 2 1/2 year old loves my old 3.0 MM. A few weeks ago when i was showing him my new WOTC miniatures he pointed to the carrion crawler and said that monster is the one in my book. I was suprised and figured he was referring to some catepillar or monster in one of his story books, but he clarified that it was from the MM. I got him the book and he proceeded to page through it until he found the entry and showed me. I was very impressed.
I think it also speaks pretty highly for the quality of the new WOTC miniatures.
My son at three only wanted dinosaurs. Then I introduced him to Godzilla.
A fire breathing dinosaur was quickly associated with my dragons.
Now, he is into Yu-Gi-Oh! (age 6) and believes his Ultimate Blue Eyes Dragon can defeat Tiamat any day.
Meanwhile- the wife is smirking watching him and I go over what each can do and why one could beat the other.
The best is yet to come-
Gaming is now held at my house on Saturday mornings. As I try to instill fear into my 1st level players with Kobolds, Tim comes with his dragons (and other assorted figures) to play also. I even made the mistake of giving him a set of 2 inch dice to "play" with on the floor.
Won't be long before I have another player in Eberron (kender I think)