What's the most leak-proof kids water bottle?

Across the reviewers we read, the Thermos Funtainer line and the Owala Kids FreeSip get the most consistent leak-proof praise in real-world backpack use. Walmart and Target verified-purchase reviewers flag that the Thermos 16 oz wide-spout can pop open if the lid isn't fully twisted, which is the most common leak complaint in the category.