Yes, Marcia's got (at least?) one child, a son. I think he was in the Thanksgiving episode and there was a bit about how Logan had put him in charge of some division at Waystar. It was apparently a surprise to Logan's kids when it came up in conversation.
It was Marcia's son that said he saw Kendall, all wet, coming back from the pond to the house and sort of forced him to cave in to his father's assumption of his being in the car. If it was just his room key card being found at the scene he might have tried to deny he was involved.