Pretty fun. Let your child teach his little brother or sister the basics from his lesson: sit tall, feet supported, hold the arm gracefully (now THAT is fun!) and TA-DA, place the hand over the keys, curved like a soft paw. See how much your older child really does know! Give lots of praise for this mentoring, and patience with the wiggly sibling.
Be silly. Find ways to count out repetitions of Ready?.........GO! for Twinkle Rhythm A. Encourage patience with the little brother or sister! Remind him how hard it was when he learned it! Rehearse the 'improper' posture as well as 'together' posture. Making it a little whacky will remind everyone that music is is full of emotions including humor.
Lucky only child. If he is lucky enough to have you all to him or herself, (no siblings) use yourself as the recipient of the lesson, or another VIP in your child's life. Being the 'shower of things' is fun.
Mentoring others is magical.