Are you familiar with the cape? We opt for further out towards Brewster or Dennis. We stay at Nickerson SP as that's our style but have friends that stay in local FHU private CGs. For little kids I'd suggest bay side beaches as opposed to the surf side, they have more fun when they can wade. For older kids surf side may be better. We toured Atlantic Oaks and give it a pass, preferred is their sister CG Shady Knoll. Where ever you go, if there is not a beach in walking distance, you have to pay for beach parking which can get filled early in the summer. We've only done cottage rentals with little kids.
If you want to stay close to the mainland Scusset Beach SP may be worth looking at, no FHU but they do have W&E and have a bay side beach right there. Still, this is a long way from the end of the cape so if you want to explore you'll be doing some driving.