When the Camelback first came out, I bought one. Mine is basically a really small pack with the bladder in it. I used it a few time before relegating it to my closet. I then bought a winterized Camelback bladder and it met the same fate - a good idea in the store, not so good in practice, for me that is.

For a kid, I think Campermom has the right idea. I would suggest a water bottle in a holder on a belt. Easy to use, low cost and if the bottle gets lost, easily replaceable. I have various water bottles I've bought or picked up at trade shows for free, but I often just use something like a Snapple bottle or bottled water bottle that I can toss in a daypack or the car and not care too much if it gets misplaced. Some bottles have the sports top you pull up to drink and with one of those, he won't likely lose the top and it is about as simple as it gets.