They'll be just fine, unless someone starts acting injured, then I wouldn't worry. My mum once had a cat who would intentionally bonk herself on the coffee table because she liked how it felt. Cats are miraculously tough creatures, so there should be little to worry about!
My cats do that, too. Play so rough! When they chase each other, they hit their heads & backs on bottoms of iron barstools, chairs, the catdoors leading to the litter box areas... you name it.
I think as long as your cats aren't acting hurt, you're okay. If they are limping or you notice swelling on their backs, then I'd worry. But cats are extremely resilient -- even into old age.
i used to get freaked out when they would chase each other on the main level of our house where we have wood and tile floors. they're just bouncing off the walls, doors, and furniture...but still they just keep going like nothing happened. i've also noticed that head butting thing with both of my cats and the coffee table...it happens to often to be an accident so i guess they like it?