Thanks for your advice, everyone. Yes, I realize she is "too big for training wheels" ... her current 16" bike is too damn heavy. My fault.

The 3 year old has a specialized hotwalk and is doing great with it. I'm looking for a 16" hotrock to upgrade her to (and also to let the 5 year old use!) so I may also look for a 20" hotrock on craigslist as well.

Off to look at the recommendations from above!