Some weeks will have daily practice with a lot of rewarding progress. Other weeks you will lose the music books, fight with your child, and feel an overall lack of accomplishment. Don't let the down times discourage you from the finish line. You have to look at the long term goal of lessons to see the reward, not the short term.
I have students who started at 5 years old with great enthusiasm, their excitement dipped around 7 to 8 years old, and then really surged again at 10 years old. The first few years we worked to develop some pretty consistent practicing habits. During the dip (and increased sports and school schedule) there was hardly any practice between lessons. They talked about quitting, but stuck it out. Around 9-10 years old they suddenly were playing more advanced music, because they were still making progress in lessons, and in 5th grade they were taking the level 3 piano exams and playing all the time!
Several of those students are now entering high school and in the last levels of piano exams. They love playing piano (even though they still whine about new pieces) and they are always willing to sit in on rock bands and other music groups when asked. Those kids became the students the other parents are envious of. But they didn't get good from constant, consistent practice. There were many times that they crammed practice right before a show or exam (and nearly gave me a heart attack!). They got good because they didn't quit, they rode the peaks and valleys of motivation and they never took a break from the race. They kept up the momentum, though the speed wasn't always the same. They stuck it out and they finished the marathon.
Focus on the finish line, and forgive the bumps and slower patches along the way. Just keep moving!