It really depends on your child’s age and daily routine. For toddlers, the kids tumbler should be small and light. So 10–12 ounces (300–350ml) ones should be enough. For school-age kids, a medium size water flask about 17–25 ounces (500–750ml) works better since they’ll need more water to last through the school day.
Keep in mind that larger bottles can hold more water, but they’re also heavier to carry. The best choice is a size that gives your child enough to drink without weighing them down.