As with bedtime, showers, dinner and brushing their teeth, kids always seem to do better with consistency. Try to set aside 15 to 20 minutes each day for them to practice. If they have homework, use their practice time as a break in between completing homework assignments - or maybe as the treat when they are finished. Regardless, try to keep their practice time at approximately the same time each day.
Just like adults, kids can easily become defeated when they feel overwhelmed. Help them choose 1 or 2 songs to work on for that day. Perhaps they need to just focus on their bow hold or getting better at reading their notes. Help them to set a goal for the day and accomplish that. They will feel more positive and enjoy practicing much more!
Each day, at the end of their practice time, encourage your child to perform at least one song for you. Not only will this keep them accountable, but it will help promote a stronger sense of self-confidence!