We did Tulum in the end of April with our DS 5. As the other posters have said, it was HOT! Friends who have visited in November say it's always that hot.
Our DS had no trouble walking around the ruins (it's really not that rough but there's alot of ground to cover) but it was a really long day: a nauseating ferry ride, a long, boring bus ride, and frankly, nothing much to interest kids when we got there except a small beach (the beach is nice and looked very clean, but it was crowded and there isn't much of a place to rinse off and change, and doing the ferry and bus again in a sandy suit wasn't appealing). My kid was a trooper but had I realized how grueling it was going to be for him, I would have sent DH on his own.
My DH insisted we do this excursion because ruins are his "thing" but I wouldn't recommend it for kids unless they are teens or budding archeologists!