Warm thighs

One can see it isn't mere sadistic pleasure, but a look of pleasure as she presses herself against the son. She feels genuinely pleased, that as the son gets stronger and stronger there is a level of fulfillment teasing him in such ways, knowing that though he is more capable of at least putting up resistance than before. While his libido has long since exceeded that of the father, she can still have him at a disadvantage, especially at times like this, and a level of affection reserved for him rises.