Have you seen the movie G-Force, the one about the secret service guinea pigs? Very cute movie, but as with any movie regarding a certain type of animal - it causes a surplus of people wanting one for their pet and within 6 months to 1 year causes an abundance of unwanted animals sitting in Humane Societies and Rescues or worse yet, being dropped off in parks and in the country.
So before you run out and purchase one of these creatures to add to your family, please keep the following in mind:
Your guinea pig isn't going to do ANYTHING those in the movie can do - make sure your child is aware of that.
Guinea pigs do not use exercise wheels in their cages or will they roll around the house in the plastic balls. So many movies have made the mistake of showing these in movies with guinea pigs. Don't waste your money.
Guinea pigs can live 3 to 7 years (sometimes longer). Are you willing to make that long of a commitment? If you can't make the full life time commitment - don't get the animal (and that goes for ANY type of animal you may be considering).
Guinea pigs do have sharp teeth and too hard of a squeeze can kill them; no small animal is an appropriate pet for small children. Can you offer total supervision while your child is holding the animal? If not, perhaps you should wait until your child is a little older.
A good rule to follow - when your child is old enough to clean the cage, they're old enough to have the animal.
Most cages sold in pet stores are too small for a guinea pigs, are you prepared to make the investment for an appropriate sized cage? Marchioro Cages are at the top of my list and optional stands are available for them. A lot of pet stores have started to carry them plus they are available all over the place online; I highly recommend the Tommy K line. A good site for cages is Cavy Cages, it'll give you information on all types of cages available in pet stores, plus show you how to build your own! When buying a cage remember - wire bottoms are a no-no (would you like to live all the time in your bare feet on wire? Neither does ANY animal).
So you've decided you are ready to make the commitment? Before running off to the pet store, check your local Humane Society or Rescue; you'd be surprised the number of small animals that are there. Check Petfinder for agencies in your local animal.