They are small and reasonable clean. They are too small in my opionin to interact well with people. The are escape artists. If you get them I suggest using the plastic cages with the holes in top so that they can't escape. With small children they should probably be a look but not touch pet.
Guinea pigs look like larger version of hampsters and gerbils.
I and my brothers had 3 guinea pigs when we were young. In other words, they were not very good pets. My wife bought one for our kids, and other than finding out all of the above, we found out that many like to pee outside their cage. Do yourself a favor, buy a rabbit.