Xcaret Park is an obvious choice..
You can kill a day.. or multiple days.. at that place with kids. It's just 15 min south of you. Tickets are a little salty.
Chichen Itza is amazing and if a good portion of your crew wants to go you can do it pretty cheap. Just know it's a 2 hr ride each way and your day will start very early. For a group that large I'd contact USA Transfers. They'll pick you up where you're staying and shuttle you there and back in a van. Don't dare step on a group tour bus.
Coba is closer than Chichen Itza and you can still climb the pyramids. It's not as grand as CI but still a great experience.
The ruins down at Tulum aren't really great.. but let's face it. You're just going for the views from El Castillo and to swim on that beach. The photos alone you get will make it worth the trip. Again... contact USA Transfers and set up something private. It'll be very reasonable. Take water and sunscreen. There's very little shade.
There's nowhere in the world like Tulum Beach. That little road in the jungle with all that high end shopping and dining... It's like nothing you've seen if you've never been. And you can drive to the end and enter the biosphere through that Jurassic Park gate. As you meander south you can stop and get out to enjoy mile after mile of the most incredible beach and water this side of Grace Bay on Turk. .....But be warned... It's one way in/One way out of Tulum Beach. Getting back to the roundabout at Av. Coba can take an hour or more if the traffic is backed up. Thurs-Sat would be a no go for me.
It's turtle season so take everybody to Akumal. You can rent snorkel gear at Akumal Bay and wade right out and swim with the turtles. No guarantees but we rented a house in Akumal a couple of years ago and swam with turtles every day.
Stop in Puerto Morelos and experience a little what a Mexican fishing village was like years ago (although it has tourist shops too). It's a world-class snorkel location. Go to Wet Set and schedule a snorkel trip for everybody. Don't miss seeing the reef that's just a mile off shore. The Belize Barrier Reef system is the 2nd longest reef in the world.
Find a map of the cenotes (tons online) and stop at as many as you can. Every one is different and lots of them are amazing.
Dine one night at Alux in Playa.. it's not crazy expensive and the kids will love it. Food is good too.
edit:.. I just looked at a couple of webcams in the RM. Damn, the sargassum is bad right now. 10 years ago we'd never seen any during the summer and now it makes us not want to go except during winter months. The people there fight it hard but it's become an overwhelming problem.
This image makes me think you should just shuttle up to Isla Mujeres and do whale shark tour. At least the beaches will be clear.