Well, I'd be more worried about the children poking and squashing the guinea pig than the children being harmed by the pig ! On that note - always ensure children are taught how to handle the pig correctly and learn they are not toys - young children should always be supervised when they are with guinea pigs - as with all animals. Please ensure the cage is situated out of a young child's reach. As to what the child could catch from a guinea pig - the main contender would be if your pig had a fungal infection called ringworm - this is contagious but your child can be treated by the doctor - as can you if you contract it ! Always follow good hygiene rules such as handwashing after handling animals etc. And monitor your guinea pigs health regularly so you spot any signs of a problem early on and can take the appropriate action - ie a vet visit / treatment. Guinea pigs are herbivores so their nice neat little poops are not really much of a concern in the way that a cat or dog's would be.