We love our stainless steel water bottle, but hadn't thought much about options for children until we came across these adapters, which turn a stainless steel bottle into a BPA-free sippy cup.
The sippy spouts screw in to the bottle for a spill-free bottle. It's made by Avent, and available at KleanKateen.com and are $4.95 for 2 spouts, and #.95 for the adapter. You can hoose from BPA-free or all steel versions.
Get 'em here.