I also checked with ADO and the schedules in their computers do not show what you are looking for. HOWEVER, it is entirely possible that when you go to the main bus depot in Playa, you WILL find a schedule to Mahahual and back. Click on the Related Gallery of photos in this article, and you will see a photo taken by friends of mine in Feb. 2009, in Mahahual, of the bus schedule from Mahahual to various destinations. If the bus does not work out, you could hire a van taxi to take you all and pick you back up. Do a google search taxis playa del carmen and you will see lots of info, or you can arrange it after you get to Playa.
Juanita Stein, YT Editor