I'm on board with "I can't believe the author showed Sou playing with herself on page 6", but I want to believe there's a relatively innocent explanation: Given her expression, maybe she's just thinking "So that's what it looks like" and touching herself for comparison, while unaware that her fingers are wandering a little more than they should for just comparison. Circumstantial evidence: Her hands are above the covers.
Man how weak and feeble are the adults in that town that a 12 year old girl who weights 88lbs is stronger than you?
My bet is they're all like MC, they won't seriously wrestle back for fear of touching somewhere they shouldn't.