I grabbed one of these Personalized Kids Stainless Steel Tumblers w/ Straw a while back from Jane and was super impressed.
It is a smaller cup (12 ounces) but really the perfect size for my daughter who is 3! I think even older kids can use this cup!
What I love most about the cup is the lid. The hole where you insert the straw is actually slanted and acts as a spout/sipping part! The lid has an excellent seal on it as well!
Here is an image of the cup I purchased so you can see quality!