BBM. That is your opinion, not fact. Adults traveling with small children on vacation should not need to stop at a bar in mid-day to get a first "cheers" drink. Their primary responsibility is the supervision and safety of the children. Alcohol contributes to supervisory neglect of children. There is absolutely no proof in this case the grandfather had consumed alcohol.
JMO
Your opinion is correct, Adults traveling with small children should not stop at a bar to get a drink. Does that mean that they don't? No, it doesn't. There isn't proof either way if he had or had not had alcohol or if he had or had not taken some kind of medication that could have impaired him. But this forum is a place where people can discuss possible theories of what happened, which is what people are doing.
SA was traveling in a group of 6 adults. He was never up until the moment mom left him alone with her for 15 min and he put Chloe out the window, the ONLY ADULT tasked with her care and safety and there is no indication that he should have known that he would be, since mom was called away on an errand from her playing with Chloe. There is no reason he couldn't have decided to have a drink or two at lunch.