How can i limit data usage on my childs lined?

I'm looking for this, too. I don't want to turn their data off entirely in this case; I just want to limit it. This one's watching movies, etc, and sharing their hotspot - which I don't mind overall. However, they're taking up a huge number of GB, after which the entire family is slowed. Everyone else in our family uses MB or just a couple of GB.

This one's doing this while away from home, and they need their phone for work/school stuff. We're three days into a billing cycle, and everyone's data is crawling because of this one family member!

I've been through all the account stuff. I can deactivate their line, say - totally counterproductive - but I can't seem to simply limit that one line's data to one or two GB/month or similar.