although i dont mind my 5 year old using the sippy that has no spout, its more like the kind of tops that coffee cups use, here is what i do.
i go to bjs and buy the 48 pack of the smaller poland spring water bottles(you could buy the bigger ones but it seems like they get wasted too much). then i save the sport top caps from poland spring bottles. i keep the tops in my bag, along with 2 little bottles. if the kids are thirsty, i open the water and put the top on. if not, the water bottle is still sealed for the next trip or for me. the sport top wont spill as fast if it falls, and it easier for the kids to close on their own that twisting on the cap.
if they are careful though, and remeber to replace the cap to avoid spills, just give them them the small bottle themselves. there are also other tops you can buy to fit on water and juice bottle tops. they are sold at babies are us and you just take the bottle and put this top on.
http://www.toysrus.com/product/index.jsp?productId=368632...
another option is to just buy the sigg bottles or equivalent and fill it before you go. the sigg for kids just twists to open and shut. my kids couldnt remember to shut it so it would spill all over. but if you are right there with them when drinking it would work great for you.