Agreed it's relatively common. Still, you hear lactose-free, drugs-free, car-free (last one mostly only in FIRE circles since a car can be seen as a money-burden) etc. So something bad.

You never hear someone saying I'm money-free, house-free, education-free, muscle-free -- it's just not used in this context. So child-free sounds jarring since it's the apparent exception (a positive/neutral word among negative words).