You can get BPA free plastic, or stainless steel. I have some Camelback bottles that I think are too difficult for a child (and they aren't completely spill proof anyway), but I mention them because they sell replacement straws as well as brushes for cleaning out straws and mouthpieces which can get moldy. We keep extra straws on hand (they can be cut to size for different height bottles) - when we're washing dishes, we just take the narrow brush and dip it in soapy water, stick it into the straw, move it up and down a few times, then rinse - straws and brush drain in the dish drainer. Same for the mouthpiece - it detaches and can be cleaned out and air dried.
So whatever you choose, you will have to clean it, and maybe run the bottle part through the dishwasher now and then. You can probably find a good assortment of cleaning brushes in a sporting goods store that sells a lot of stuff for runners and other athletes who are always toting these things around and leaving them in gym bags.