If there's one thing that is guaranteed to get your kid enthusiastic and interested in what they are doing it will be if YOU get enthusiastic and interested in what they're doing. By this, I don't mean for you to take over their practice sessions, but to gently offer your support and interest.
Ask your child to teach you something they've just learned and make them feel like they are in the driving seat. Show that its ok to make mistakes and that this is how we learn.
Get them to give you a little concert once in a while and offer praise for their hard work afterwards. This helps to put playing music in context for them and helps them to see that they can bring other people pleasure through their music making.
If your child practices for just 10 minutes a day and they are focused and committed to what they are doing with no distractions then this has been time well spent.
If your child spends half an hour on their practice but is distracted or thinking about other things, or feeling like they are being made to practice, then it's highly likely that when they practice again, they will not have remembered what they have done and feel like they are not progressing.
10 minutes is also much more manageable for those children who are reluctant practicers and, miraculously, their concentration and willingness to show up for their practice often increases over time as their love for music grows.
Try using a timer so that they know that there will be an end point to their studying. Quite often children will begin to 'graze' on their practice and will return to it a few times if they know that they can set a timer and mix it up with other things that they like to do.
As Mary Poppin's informs us, when we have work to do, the more fun we can make it, the better it is.
Rather than giving your child a spoonful of sugar though, I recommend turning your child's practice into a game.
Make charts with stickers for practice achieved if that works for your child or create an award system where you have a jar on top of the piano and they receive a marble to put in the jar for each practice they complete. Once they've filled the jar they get a music related reward. You'll be surprised how often your child will be returning to their instrument as they love to physically watch their pile of marbles growing.
If your child is struggling with practicing scales, make it into a board game, if your child can help to make it, then even better. Use one square per scale and mix it up with other more physical tasks like hopping on one leg or running on the spot. Get imaginative and tailor it to what ticks your child's boxes in the fun department. If your child knows someone else that is at a similar level, invite them over to play the game. Learning is way more fun if you're doing it with others.
One of the most important things you can ever do to encourage your child with their music making is to play them as much music as possible.
Find music that is specific to your child's instrument and play them music from different genres so that they hear their instrument in different contexts. This will help them to see the potential of what their instrument can do and how they can develop their own playing.
Try making playlists for the car using pieces that your child may be learning played by lots of different musicians. Its easy to find hundreds of different versions of traditional tunes on you tube that get used in lots of instrument method books, like Scarborough Fayre or Amazing Grace.
Finally, if your child still needs encouragement to pick up their instrument, get together and have a family jam session. Even if you can't play anything, grab a saucepan or colander from the kitchen and bang out a beat to whatever they're practicing. Try to look on this experience as a way to encourage a life long love of music and learning that you can both share and enjoy!