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!
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?