Setting a certain time aside to practice may be the key to progress for your child and for yourself. For the child making piano practice into a routine communicates the importance you place on this endeavor. It also gives your child the security to know that what happens today on the piano bench can be improved and enlarged tomorrow.
As the parent having and staying with a practice routine will give you confidence that your child will learn to play. Let's face it, parents carry much of the guilt for failed improvement at the piano. I know when I took my daughter to her lessons I felt very bad about the days that disappeared without a thought to her playing. Realistically it may have been her fault for not going to the piano, but I know it wasn't until our routine was set that I could expect her to be responsible.
This summer I had a incentive to keep practice happening. I rewarded every 100 minutes with a scoop of ice-cream on a cone. For some students this was ridiculously easy, and I think it was because they already had a very set routine of daily practice. Except for the very earliest beginners, every child should practice at least 20 minutes a day. And it needs to happen most every day. Research shows that the brain needs the repetition every day and it shows that cramming for a longer period once in a while does not prove the best results.
I tell my students the story of what I learned from my third grade teacher. She applied this to learning spelling words but it works exactly the same way in piano practice. She told us to go over our spelling list in our heads before falling to sleep on the very day we received the new words. I experimented with her advise and found it to work. So, if you ask your child to go through all the music he learned at the lesson before bedtime, the remembering will place the information in longer term storage. This need not be a long intensive practice. It is more a remembering and it would be ideal to sit with your child and ask questions about his new pieces. Your child may want to skip practice on the day of their lesson, but this is the very day that practice is the most beneficial.
"A great deal of research shows that thinking or talking about an event immediately after it has occurred enhances memory for that event." This is written about in Brain Rules by John Medina.
Think about how confident your child will be the next day when she goes to practice and her assignment is familiar. Children have an "I can do it " attitude naturally and when they find they can't they sometimes assume they have failed. They are much harder to motivate in failure mode than if they feel they are successful. So, when is practice time at your house?