Carry my .5L SIGG with me in the parks clipped to my bag at all times. No need to add more plastic bottles to the waste stream. It doesn't weigh much empty, nor does it take up much room. Yes, you can get ice water for free at CS locations, but they aren't always convenient. Kids might do even better with some of the collapsible ones. I'd look for something relatively cheap in case they loose it, but good enough to continue to be used at home for other trips, etc. No harm in teaching them both responsibility for both their own stuff and the planet at the same time.