I've had multiple guests ask me about gratuity practices for group lessons for kids or adults as well as private lessons, so here's this instructor's take on the subject.
When I've purchased lessons for my better half (Johanna), my snowboarding brother-in-law, Gary or my tentative sister, Susan, I book only the best, most experienced, and most entertaining instructors on the staff. These include Eddie Visser, Randy Bell, Matt Majersky, Jeff Hickel and Mary Ellen Pearlman. I tip these folks at the 20% level, unless Johanna or my sister beats me to it!
As a general rule of thumb from my experience on the receiving end, a $50 tip is a fine thank you for a half day lesson, and $100 is similarly appreciated for a full day private.
Let me take this opportunity to thank all of the parents who trusted me when I was a kids' group lesson instructor; and, of course, all who've gone on to choose me when private lesson attention is desired.
Thanks again, and I hope this post is helpful to folks new to skiing!
Guidance from other sources on the web:
From Trip Advisor: Tipping your instructor
From Park City Resort's Website: Should you tip your ski or snowboard instructor?