K.W.
I use tiny glass mason-type jars for my kids. The 1/4 pint jam jars (4oz) are a convenient size for snacks and are absolutely leak-proof when tightened down properly. The lids have also been a standard size for many decades!
They can break, but not easily. I've dropped empty quart-sized jars 3 feet onto concrete and had them bounce. The tiny jars are even tougher. You can find them at most grocery stores and hardware stores.