If you go further south, my daughter and I just went through Aktun Chen. It is down by Akumal, not as far south as Tulum. This is a series of underground caves, the last having a beautiful grotto in it. Tour takes close to two hours, includes a guide, for $22. My daughter is a geologist, she was fascinated, so was I.
http://www.aktunchen.com/
My kids and their significant others spent a day at Hidden Worlds. They have canopy tours, swimming, snorkelling or diving in the cenote, they made a day of it.
http://www.hiddenworlds.com/
That might be a bit too far to go if you have little ones. If you want to see any ruins, from Puerto Morelos, I would suggest Tulum for sure, and you can combine that site with Coba in one day. I would get a guide if you go through those architectural sites, you just learn so much more than walking on your own.
If you like fishing, the best is down south by Boca Paila, in the Sian Kaan preserve. From the town of Tulum, head east to the ocean, then south along that coastal road for about half an hour, easily travelled by any vehicles these days. My husband and son hired guides a couple of days, they caught bone fish, snook, some jack thing? barracuda, but no tarpon, which is what my son was really after. You can also do interpretative jungle and coastal tours through the Sian Kaan Preserve.
I don't know the ages of your children. My feelings as a parent when my children were smaller, give em as much beach as they want, they are happy campers beaching or in the water. Crabby kids on tours never really turned my crank, I left that until they were old enough to decide if they wanted to come along.