Well we have two who are both keen on playing and doing grade and play in public pretty often. The elder is very self-disciplined, gets up early, and does her twenty minutes before breakfast. The younger doesn't like to practice at all, and needs careful steering towards the music stand. Having a target such as an impending gig or exam helps, but it's tough.

So, I guess I'm saying that it seems to be either in their nature to practise or to not practise. Then again I suppose seeing that their dad (me!) practices lots must help by example. Except I don't call it practicing.