My 4-year Old Granddaughter Has Recently Been Placing Her Head

My 4-year old granddaughter has recently been placing her head in the crotch of my 8-year old grandson. This evening, my grandson was trying to push her away saying that she was 'licking' him. He had underwear on and was laying in his bed. She was supposed to be giving him a good night kiss. l'm concerned that she may have been sexually abused or perhaps was witness to a sexual encounter. is this normal? what should I do? Her parents are divorced and both have significant others. Both her parents have sexual issues (father raped mother among other things. mother is currently sexual deviant as well and alcoholic). Thank you for any assistance you can provide.