Younger kids especially can lose focus easily when they are tired. I can't teach kids too late in the evenings or they literally cannot focus and sometimes break down into tears. You can't make a kid focus when they are in this state, so pick your practice times wisely! Just after school is perhaps another time when they might be in an energy slump, they need a break and food - depending on the child of course. This leaves early evening and the mornings for practicing. With increasing demands on homework for school, It might be effective to try making it a habit to practice before school in the mornings when their minds are alert and fresh. I woke up and did this in my school years, and now the students of mine that are keeping their practice habits are finding this works for them also. There's the added advantage that its then done for the day and they can relax and focus on their other activities when they get home from a long day. It doesn't have to be a very long session, but it starts getting them into the habit, making progress and when they see the results, it encourages them further. Try it!
Happy practicing :)
Here I write my musings on teaching, or other things that come up in lessons/relate to violin learning. Anything suggested is just that, suggestions and my thoughts and may differ from the opinions of my other music colleagues!