If you notice your child's mood change for the worse or if you have been working on a tricky part of the music and you feel he/she has is ready to stop it's best to cut your losses and consider bringing the session to a close. Here are a few suggestions as to how you can raise your child's mood and end on a positive note.
Try ending the practice session by suggesting your child play an old favorite from earlier in the book or another book. It doesn't matter if he/she makes mistakes, the purpose of playing it is to simply have fun.
If you are feeling adventurous you could clap or sing along to it. You could take a couple of photos/videos to lift the mood while your child plays...children often love posing for the camera and looking at the pictures afterwards.
Suggest performing the piece to another family member before finishing the practice session.
It's important to remember to allow your child to play the piece that he/she is working on all the way through before you stop to point out errors, it helps to give your child confidence and also allows you spot where the weaknesses are. You can then address them one at time and work to improve each one.
Don't forget to offer to point to the notes as your child goes along. If there is resistance, suggest that you point the first time through. He/she can play it a second time without you pointing.