Having owned both types of animals, I'd suggest a hamster instead. They're smaller; thus they live in smaller cages, require less frequent cage cleanings, eat less, and are generally less trouble -- a consideration for a child who's going to learn to care for an animal for the first time. My guinea pigs needed their cage cleaned 2 Xs a week . . . my hamster always "goes" in one spot in a removable portion of the cage; so the main portion of her cage can be cleaned every 2-3 weeks, and the little removable spot is changed about once a week. Big difference in cleaning effort.
Hamsters also move around and "do stuff" more than guinea pigs. They run on their wheels and exercise balls, etc.
If you decide to get a hamster, consider a Teddy Bear hamster (or one of the variations: Black Bear, Panda Bear, Honey Bear -- they're also called Syrian hamsters). They're larger hamsters (easier for kids to hold), and they like people, whereas the dwarves like each other. Dwarf hamsters should be housed in pairs; Teddy Bears are territorial and should be housed individually.
Don't get me wrong: I think guniea pigs are great. I just like the hamsters better!