REI passage 40. My daughter is 8 in the photo. I got it on clearance, plus their 20% off coupon. The bonus being it is a neutral color so I can pass it on to my son later on.
I tried a Camelbak hydration pack on my 4 year old and it didn't work at all. I assume you guys recommending them are talking about a kid sized one? I put one of mine on him and even with the straps tight it hung down to the backs of his knees. The shoulder straps wouldn't stay on his shoulders as they were too wide.
I think those Deuter packs look good for the 8-10 year old range. Might have to use the REI Dividend for one this spring. I've had good luck using the REI brand packs for the younger kids. They usually have 2-3 sizes, and REI is not far away, so I can take them down and make a production out of the purchase. This year its my daughter's turn to get a pink one vs. the blue or red hand me down from big brother.