i always use a camel back when i'm outside for long periods of time. you can get them at a Bass Pro Shop, Cabelas, Walmart and Target even. Walmarts are knock offs though, but thats the one i have, they run in the 30 dollar range and are great! its a big water bottle backpack thing that has a tube that runs over your shoulder to let you drink out of it. Make really sure you keep it clean though, great place for bacteria to hide. take it apart and clean it every day if you get one. they hold about a half gallon of water when they are full and when i'm using it i try to drink all of it twice a day. Also this is kind of gross but you know that you are well hydrated when your pee is clear or close too it, yellow is bad when you are outside a lot. Make sure your child also gets plenty of salt which gets more difficult because you can only put water in a camelback unless you want to replace the bladder after each use and they're about 5 bucks and they should be replaced about every month anyway.
Another thing i use is a nagalene, its basically just a clear bottle but its nearly indestructable, dishwasher safe, i use a clip and keep one with gatorade clipped onto my camelback. you can get both the clips and the nagalene at walmart also over in sporting goods, the nagalene also has knock offs that are only about 3 dollars and the clips (caribeners) can be used for so much that its always good to have a few of them around anyway. they come in several colors and sizes and are about 50 cents to a dollar each. if you get all this stuff its gonna be about 40 dollars and if you're like my family you gotta watch your money but its a good investment. if you really wanted to you could just buy a camelback bladder and stick it in you daughters favorite backpack but the camelback bag part is nice because it's pretty small