For fucks sake a baby or toddler won't know what it's parents are doing.
They won't suddenly now to be disgusted because Daddy is screwing Mummy not tickling, hugging or kissing her. Assuming parents are under the covers and not having oral sex in front of a toddler, the child will see nothing that will bother them at all.
No doubt they hug their parents and older siblings, climb on them and bounce on them.
My guess is the child will remember nothing at all because they see nothing odd, scary or different to make the memory stick. I'm told I used to wander in on my parents, I certainly don't remember.