As a Japanese, I would like to say the priority seats are not only for people with challenging. When no seat available, some people have a priority to use it. For me, I always stand up to offer the seat if I find people with challenging, not only at priority seats but also at any seats.

Can a mother with a young child sit there if the seat isn't taken after everyone gets onboard?

I don't say so. They are literally "priority seats". If there are no such people around, you have your freedom of take seats same as everyone and in any seats. Just these are prioritized seats for those people who have disability, old, pregnant or with children. Priority is priority not exceptional. Probably i would like to draw more attention on be careful not block people with suitcases. And so in everywhere not only around these seats.