I would tell them whenever they're sitting at home, sit in pike and use one hand to push down on their knee, and the other hand to pull their foot up flexed (so they're trying to keep most of their leg down and just get the heel/ankle pulled up off the ground). I had a friend with a similar problem and I used to have much worse pike flexibility, and we started doing this a lot around the time we were L6 and had a lot of improvement. I still do it all the time.
Also on bars they can do straddle and pike hang with their toes up on the bars, trying to straighten their legs.
I think if when they stand up naturally, unless their legs are bent (not usually the case) they can work on it. I think this is a strength and flexibility issue in that where their muscles naturally are going when there's not the same force as when they're standing (not sure how else to say it, I don't know if that makes sense) is to contract more, and they don't have the strength to push against it. If they were more flexible they'd need less strength to push against it. Some people just have much tighter muscles. I'm not an expert though so that's kind of just my guess. But I'm pretty sure it can improve at least for some because I had a friend who had the knobby bent knee look as a younger child and went on to become a high level rhythmic gymnast. She had to work for it though.