It's a struggle, isn't it? Most kids don't like to practice, even if they love music and their lessons. It's very rare to find a child who will practice without being told to do so, and if your child does, count your lucky stars!
Here are some things you can do to help make practice a positive experience:
1. Set aside a regular practice time each day and do your best not to schedule something else during that time. Perhaps it's as soon as homework is finished. Maybe it's before school - a lot of young kids do very well practicing in the morning when they are less tired.
2. Minimize distractions. Make sure the environment in which your child is quiet, with no telephones ringing, dogs barking or siblings interrupting the practice time. Kids are so easily distracted and we can help by creating a practice environment where focusing is easy.
3. Don't shut the door at practice time. Listen when your kids practice. Even when it isn't the most beautiful thing to listen to, it lets them know that you appreciate their efforts and encourage the learning process. When you hear things you like, let your kids know it!
4. If your child is young, intervene. I recommend any child 7 or under have their parents present at all lessons and during practice. As they get older, you can gradually ween yourself off of practice duty as they are more capable of staying on task and organized. Your main responsibility is to ensure that kids stick to their assignment sheet. All of my students have a notebook to keep track of this. You should be reading this notebook and ensuring that what's in it gets covered. Kids like to skip the items they feel are less pleasant. It helps to have them read the notebook out loud to someone at the beginning of their practice daily, even if they think they already know what it says.
5. Encourage them to play around. It's always okay for kids to do some improvisation and to explore their instruments. Occasionally, it results in some less than pleasant sounds, but pushing these limits helps them discover what the instrument can and can't do. Just make sure that play time isn't all they're doing. They still have to complete their assigment!
6. Reward them. Create incentives for practicing regularly, staying on task, reaching goals, performances, and especially for overcoming particular challenges. Perhaps their practice time earns them some video game or television time, a meal out to eat, a trip to an amusement park, or a trip to the music store for a fun musical toy, game or a new piece of music (Frozen is particularly popular at the moment - ask your teacher which version is best for your child's level).
The most important thing to remember is that it is your responsibility to ensure your children practice and that their assignments are completed. It's not reasonable to expect them to shoulder this responsibility themselves. It is not in most childrens' natures. If you're having trouble with this concept, put yourself in their shoes. Would you have done it on your own at age 5, 6, 7, 8? Neither will your child!