I would add to the concerns of sun protection and shade, the need for constant hydration. I am not an expert, nor do I know what the medical community says, but when we took our 6 month old daughter hiking in Arhes NP, we have her water in addition to nursing just to keep her hydrated in the desert. Nursing gave her the calories and meals she needed, but I felt like the added water helped her stay hydrated. We also kept her covered and protected from the sun. I think we discussed it with our pediatrician before hand too, but that was many years ago, and my memory has faded.
With older kids, we take crystal light, and make that up alot. We enjoy having flavoured beverages over water. Nalgene makes a 1/2 gallon rectangular jug that works really well for it.
"If I'm not there, it means I'm dead...or late!" General W.R. Monger