Strategies will definitely need to be adjusted according to age and/or stage but I think regardless of age, there usually comes a point where children need to hear that practice can be boring, frustrating and make them want to give up. Hearing that not every musician enjoys practising is useful, as is the information that sometimes it's a case of needing to just get on with it so the effort can go into the music rather than into complaining about practising.
What @Bobkitten said about being explicit about what practice looks like is essential. Saying 'practise bars 4-8' is useless and can be quite demotivating whereas a more specific instruction, 'play bars 4-8 tongued at 85bpm and increase the speed a little bit each time until you get up to 100bpm...' is more likely to be followed. Having a definite end goal for a task can be good for children who like 'to do' lists. Make sure the shape of a practice session is clearly described too e.g. warm up by doing blah blah X times then play through your favourite Y. Spend X minutes on playing through Y scales and choose one to focus on. Work on the speed/fingering/articulation of today's focus scale until X....
It's the sort of thing that probably needs to be provided to the child and to the parent as a hard copy so parents are clear on what you consider practice to look like. My DCs like to have the freedom to only do 'fun' music in some of their practices. 'Fun' is anything they're playing because they like it or have chosen it rather than something they're preparing for an exam/assessment. One of my DCs likes to do a couple of 'fun' sessions each week whereas the other needs that less.
Recognise that any playing is a positive and if that means slower progress through grades, it's not a problem. If a child chooses to play their instrument for fun, that's a good thing. Ergo, if a few weeks need to be given over to playing Coldplay on the viola, so be it. Involve the child in conversations about what they want to be able to do on their instrument. If they want to be able to play jazz trombone and to improvise, follow their interests and set aside the Mozart requiem. After all, it's possible they'll come back to the Mozart in the future but if they give up the trombone at 13, they might miss out on the jazz and the Mozart.
My DC both respond to things like the #100daysofpractice challenge on Instagram, a Pinterest-worthy habit tracker or the Andante practice app. Because they're my own children, I can offer incentives that teachers can't e.g. if you manage to do 100 consecutive days of unprompted practice I will buy you a pair of the overpriced joggers you've had your eye on. Being able to communicate with parents re: a suitable incentive for a long-term goal might be worth considering or you might be able to offer a sideways incentive e.g. after 50 days of unprompted practice, we'll spend a lesson making working instruments out of junk/I'll teach you how to play the