It can be hard to get the kids to practice their instruments. Sometimes they're having trouble focusing, don't like their assigned piece, or are just grumpy. One thing I'll offer them in these cases is "busking practice".
The idea is, we'll pretend we're busking together. I'll start playing a tune they know, and they should do something that sounds good. That could be playing the tune with me, making up a harmony, or just figuring out a single note that sounds ok and playing patterns on it. If they make a mistake, we keep going. If they can't figure out what to play, we keep going. We're practicing performing. It helps that they're pretty motivated to get good at busking, because they know they can earn money that way.
Working on the pieces the teacher assigns (if you have a good teacher!) is very efficient a turning time on an instrument into becoming a better musician. But if willpower is the limiting factor and not time, and especially if the alternative is a super short practice or no playing at all, the important thing is just picking up the instrument and playing something. I like that busking practice give us some structure for this, and lets the kids build up their performance skills.