This would make for a bad way to find out your young child wasn't a virgin. Think about it. You think you're going to snap a really cute picture playing on the fact that your child is a virgin, but they hesitate and say, "well, dad, actually..." All of a sudden that road trip feels like it's a hundred hours longer than it felt like before. "Should we still take the picture?" / "No, dad. No."
At what point is it appropriate for parent to make fun of their child for being a virgin? There are more years where it's appropriate than years where it's not. I'd say age zero to eighteen are definitely off limits. Eighteen to twenty five is shaky ground, but after that it's fair game. Twenty five into the discernible future is a way longer time.