I think you mean Club Cantamar, which is in La Paz. If so, you should think about flying to La Paz, rather than flying to Cabo. I have not been to Club Cantamar, but the guidebook reviews like it. I don't know what the beach right in front of Cantamar is like; I do know they have a dive op.
For Cabo, August is low season. You're unlikely to run into spring break college kids. If you stay "far away" from Medano Beach in Cabo, you will absolutely not run into spring break college kids.
Depending on your budget, you can try the Pueblo Bonito Sunset Beach (pacific ocean side of Cabo). While your family vegs in the pools, you take the shuttle to town & go diving. Or, you could stay in the Riu Palace (new architectural monstrosity on the north side of Medano Beach, fairly far from town, but still walkable), or Villa Del Palmar (a bit closer to town than the Riu Palace). The Riu is all-inclusive, and has an onsite dive op. As they opened pretty recently, you could probably get a deal there. Villa Del Palmar has a great kids' slide pool, and a number of kids' activities.
We've been staying at the Hilton Los Cabos (about 20 km from San Lucas). They have a great kids' club program where your children do crafts & such. Not perfectly deserted beach, as there are some jetskiers & ultralights, but not obnoxious. You can then either take a shuttle or bus to town, or rent a car. If your family golfs, the Tourist Corridor is where you want to be; world class golf courses next to the Westin, Hilton, & Sheraton.
