Even with the crowds, the walk up the waterfalls is lots of fun. Guides helpfully carry cameras and make time for photo opportunities, like this one above. Be prepared to get soaked! Bring a waterproof camera.
Plenty of kids make the climb up the waterfalls. Seven is perhaps a good minimum age, depending how sure-footed your child is.
Bring aquasocks, if you have them; rentals of these handy footwear items are available, but cost nearly as much as a new pair at home. My nine-year-old did fine wearing his sturdy rubber sandals (which had firm tops and a back sections around the heel.)
For a quieter waterfall experience, try YS Falls in the southwest about an hour from Negril, or the beautiful Mayfield Falls.