Concussions are bad news, for kids and adults. There's actually plenty of research showing that kids are more likely to bounce back from certain brain injuries than adults are, though. Their brains are still growing so actually have the ability to "send" a function lost to brain damage over to another, uninjured section of brain (Neuroplasticity - Wikipedia, the free encyclopedia). Buuuuuuuuut, that being said, definitely put a helmet on your kids. That kind of stuff happens to varying degrees (here's an interesting example: Hemispherectomy - Wikipedia, the free encyclopedia), so yeah, a lot of irreversible damage can still occur. I'd suggest one for yourself too; your brain is you, you should protect it.