I wanted to elaborate on the issues since they were numerous and might help you nip them in the bud.
Let me preface this by saying I love my father in law. I'm lucky to have as great a guy as him. It's why we wanted him to join the trip.
We invited my FIL because he's older and "You never know" if this would be the last trip. He's spritly and lives alone in his house, so by no means is he some feeble old man in our minds. We did not invite him to take care of our kid. We paid for his plane ticket and he stayed in our hotel room. He just needed to pay for his park ticket.
Issues that popped up in no chronological order:
He would just walk away. He didn't have a "stick in the group" mentality and would just wander off. He keeps his phone in his sock so when we would call him, he wouldn't answer.
Left to use the bathroom. Came back 20 minutes later angry because he couldn't find one. I asked if he went to an employee and he said he did not get help. I then walked him to all bathroom breaks (but on the sligh so he would not get upset being helped).
He could not figure out how to get from Star wars land to toon town even with a map. He got angry when I wouldn't tell him and asked him to figure it out.
He got lost going from our hotel (across the street) to meet us in downtown Disney. It was raining. He said he got lost, but I think he just wanted to go explore and used it as an excuse because my wife talked to him about walking off.
He would tell us something he felt we wanted to hear "I'll meet you in 30 minutes." But then do his own thing and leave us waiting.
The day got very hot and our 2yo son needed a nap and to cool down as he was over heating. We asked him if he wanted to take our kid back to the hotel and he said yes. Later we found out he had side trips on the way back to the hotel instead of going directly there with our son.
He put zero effort into planning or preparing for the trip. Expected us to be tour guides but then disagreed with plans.
Fought us on him getting a mobility scooter after the first day. (He later thanked us for it. Game changer.)
Again, this guy showed no issues before hand in terms of capability. He can prep a boat for sea and all other kinds of stuff but for some reason travelling made him act differently.
We've gone on trips with him later knowing what he's like and things have gone so much smoother.
Now onto the baby issues!
Getting back to the hotel for a nap in a cool hotel room was quick! BWPPI is so close that you only lose an hour or two when hitting the hotel. Our son would fight the nap so hard! But because we were so close, once he woke up we were right back at it.
Did the baby mess himself? Quick trip to hotel.
Forget something? Quick trip to hotel.
Buy a bigger item and don't want to hold it all day? Quick trip to hotel.
Kiddo just having a bad day and fussy? Quick trip to the hotel for cartoons.
Letting one parent stay while the other took the baby? Easy quick solo walk to hotel.
Since we were at BWPPI, we didn't need a vehicle. No paying for parking, worrying about damage to rental car, saved money on renting.
Getting picked up for the flight home at the hotel? Go spend some time in downtown Disney on your last day. It's only a few hours but it makes leaving easier.
This got long, but just expect the unexpected. A close hotel makes it easier to deal with these things.
Disneyland hotel is crazy expensive and actually farther away than BWPPI which is why I always go there.