There are many hydration packs out there that are great for kids. The best options are either vests or full backpacks with chest clips so that the pack doesn't accidentally slip off of your child. You also need to make sure the pack is lightweight and doesn't have too large of a capacity, as kids won't be able to physically handle an adult-sized hydration pack. Make sure your child is capable of carrying the weight of the pack you're considering for them before purchasing!