Just got back from Playa Del Carmen and Cozumel and have been there in the past as well.
Things to see and do.
Chichen Itza
One of the 7 wonders and an amazing place to visit. I was there back 1998 when they still allowed people to climb the buildings and go inside the main ruin to see the jade jaguar. They are trying to protect the site more and no longer allow you to climb or go inside the buildings but it is still amazing to see. Sounds like it might be in your path and it is worth the trip. If you are driving you will be able to spend a good amount of time there compared to the tours that only give you a couple of hours.
Cenotes
there are many around the area you will be in, they are worth a look.
Xcaret
It is an eco ish amusement part of sorts... Kind of hard to explain. You can go snorkeling in an underground river, hang at the beach, snorkel at the reef, visit a small Mayan ruin and they have a show at night that that is worth watching. This place takes an entire day to visit. Not sure how it would be with a small kid.
Tulum
Nice ruin on the beach, it will take about 3 or so hours to see the ruins. If you are driving and want a guided tour just wait for a tour group to arrive and follow them. You can also just walk around and read the signs. Bring your swimming suit, lunch and beach stuff with you and make a day of it, there is a nice beach to relax at when you are done looking around.
Xel-HA
I have not been there but I have heard that it is a nice place to visit
Ferry ride from Playa Del Carmen to Cozumel & things to do (rushed 1 day or easy with 1 nights stay)
Ferry ride from Playa Del Carmen to Cozumel $12 US.
The main Part of Coz sucks so plan on renting a car and drive around the island. Once you get away from the docks and around to the other side Cozumel is really nice with miles of beaches with no one on them, you can walk down the beach and find tons of shells and just enjoy the nice water away from everyone.
On your way around the island you can visit the following
Punta sur (cool lighthouse, nice beach and decent snorkling)
Tequila tour with free taste testing, they will try to sell you a tour but it is worthless just go inside and tell them you are interested in buying / trying the tequila and you will get the same thing for free.
San Gervasio Mayan ruin it is small but worth seeing.
Tips for Mexico
Do not leave anything lying around or it will be gone, put it in a safe when possible. Also consider putting you money in more than one place, for example some in your wallet and some in you pocket or waist belt.
Make sure you always have gas, when you get away from the main areas gas is not always easy to find
Call your cell provider and see if your phone will work down there and ask about the charges. I always have mine with me just in case but have it turned off. It has saved me a few times
Pack Mosquito spray with as much deet as possible, this time of the year you will get torn up if you do not have it on. I would not recommend the natural stuff, the person I went with last time told me they had bug spray and not to bring mine. Unfortunately the one they had was all natural and contained no deet and I got eaten alive and went the next day to buy some with deet at about 10X what it would cost in the US. Not sure if deet is ok for kids?
When you are trying to buy something it is always negotiable, they know people are going to try and haggle with them so they set the price at an insane level to start with. I personally hate haggling and have found that if you ask how much something is and just say no thanks they will keep dropping the price. Once they say that is the lowest they can go tell them thanks and start to walk away chances are they will drop the price even further but if not that is probably the lowest they will go.
One more thing... I went on a horse back ride down there and came back with ticks all over me, I spent hours getting them off and it sucked so be aware of the ticks if you go out into the brush.
