Material: Opt for BPA-free plastic or glass. BPA-free plastic is lightweight and durable, while glass is free from chemicals but may be heavier and more prone to breakage. Size: Choose a size that is easy for kids to handle and fits well in their school bags. Typically, bottles ranging from 12 to 20 ounces are ideal for kids. Leak-proof Design: A leak-proof cap or lid is essential to prevent spills and messes in bags.