It's not really the value of the machine, that I see as the major problem. Children have a habit of blaming their parents when they get hurt. So I provide the bike and the opportunity to ride it. He eats a Semi at about 70mph. Wonder who my daughter might decide is at fault. I doubt it will be real good for our relationship even if she doesn't outright blame me.I say let him ride one. The realtionship with him and your daughter are worth far more than any machine, two-wheeled or more.
I've loaned a bike and I've ridden others' bikes. I even let BirdDog park a rental car in my driveway and take my spare bike for a 3 day business trip through MA/RI/CT.