So I was just thinking back on various cenote trips when the family was visiting - and not to mess with your plans, but I'd consider going to Gran Cenote instead. I've taken family along to snorkel while I'm diving at Dos Ojos, and it's not nearly as nicely set up for snorkelers. I've dove it a zillion times, and I'm not sure I could point you to where the various snorkeling entry points are. (Of course if you're guided, they'll know).
But really, Gran Cenote isn't a lot further (it's just inland a bit from Tulum on the Coba highway), there's no walk in, and it's really really beautiful - not just the caverns, but the layout - I take visitors there all the time, pack a lunch and hang out for a couple of hours. There's a beautiful little overhang and grotto area that's straight out of a movie set. For snorkeling only, it's definitely my first choice.
Wherever you're going, take bug spray.