She had an old NRS youth PFD way-way back, then wore a neoprene shell type III waterski PFD until she was about your son's age/size, then she either stole my wife's old Lotus Lola, or wore a small adult Stohlquist (again, shell was slightly too big)
She saw the Ninja on clearance last summer. I didn't grab one then for $59. Then with 2 months to go until Christmas, couldn't find a S/M one for under $100. Ended up buying her a L/XL for ~$100, but I see they're mostly back for clearance prices now. I'd have expected they'd have another version out on the market by now if they were clearancing those models at the end of last season.