i hoist my kids up to the sink, let them rinse their brushes and put them away. then i cup my hand and kind of... squish water in their mouths? that sounds awful, but they actually love it! they've got the spitting pretty much down now, i guess it's just instinctual. if they were using a cup though, they would definitely just drink it.
anyway, that's what i do. my kids are ~20mos