I love these Funtainers, but they're still not completely kid proof. When my daughter was little, she was always leaving it open on it's side and it would leak on the floor. The other issue I have is that the image scratches off if it is bumped around enough.
I will say, make sure you take out the clear insert and thoroughly clean it with a straw cleaner. If not, as with anything, mold will grow. Also, make sure it is put back in snugly, otherwise the cup will really leak.
With that being said, these are still great thermoses. The flip-up handle is our favorite part, because we can secure it to her lunch bag or stroller with a carabiner clip.