Assuming passports are in order, I can't think of any restrictions unless you plan to do the road trip with a rental car (I don't know if any Belizean car rental companies allow vehicles to cross into Mexico).
So that raises the question of how you intend to travel -- bus or transfer service or water taxi from the cayes to Chetumal?
When leaving Belize, the departure fees amount to BZ$40 (or US$20) per person.
When entering Mexico, an FMM tourist card is obtained at the border. The Tourism Tax of 500 pesos can be paid at Cancun airport before checking in for your flight.
Is there anything specific you are wondering about?