I will say that I disagreed with a family friend who made my daughter turn in a $10 bill she found in a parking lot at an arcade restaurant establishment. It was exciting for my daughter but the friend made no provision for her to claim it if no one asked, and it most certainly became a windfall for the employee who took it from her.
That friend is a bit of a ditz, so it simply didn't occur to her, but for a small child, the thing should have been thought out a little better.