Just gave my 4-yr old her first bike for her birthday this weekend. I debated the question of x-mart vs. LBS vs. used long and hard.
Ended up buying a 16" Schwinn from Performance.
I would have looked for a used bike, but, since it was her birthday present, I couldn't bring myself to do that. And, I've had such bad experience in the past w/ X-mart bikes both in overall quality and assembly, this gave me a bit more piece of mind.
Dan