Hi Jill,
Just saw this other post of yours! Chankanaab is probably about 5 minutes down the road from Paradise Beach if not less. I know we passed Chankanaab when we were on our way to Paradise Beach. You could easily do Chankanaab until 2pm and then go to Paradise Beach. I have heard though that the beach at Chankanaab is very nice too although I haven't been there myself.
Hope that helps! Better late than never!