Tulum - both the beaches and the Tulum Maya ruins - are my favorite. Stunningly beautiful. Coba is another Maya ruin that's not as touristed as Chitchen Itza and interesting (it's close to Tulum). There are also many "cenotes" (literally sinkholes in the limestone filled with water) which are an unusual attraction and great "cool off" spots. They have no safety features though (it's at your own risk) and the wet limestone ground is quite slippery, so I'm not sure they'll be great for 8 year olds (but they're really fun for adults). I would wear some kind of water shoes with some traction if you go to prevent slipping.
There are some sites that are more accessible/closer to Cancun than Riviera Maya, so it depends where you're staying exactly. I'm guessing Playa del Carmen?
(PS. Mexico is in América del Norte/ North America, along with other countries)