Even at my age, the first couple of nights I spent in the TC I woke up completely disoriented, at least until I tried to sit up and wanged my head on the ceiling.
Trying to imagine what it might be like for a 4-year-old on his first real "sleepover" without the parents being right there... or a ceiling to wang his head on to bring him to his senses.
He's stayed at his grandparents' but mom & dad were always right there, and in full view when he stood up. If I'm in the cabover bed, he won't see me in the dark and is liable to panic if disoriented badly enough.
If this ever actually happens, I may just put him in the cabover with one of those bed rails, and take my lumps on the dinette. Bring my smartphone and a long USB cable so I can play Angry Birds all night...
2002 Chevy 3500 DRW 8.1L/Allison
2000 Palomino B1500
...and the reason why I need a DRW to haul a Palomino:
2004 United 7x14 tandem axle enclosed toy trailer
2011 PJ 8x20 7-ton deckover equipment trailer
2 years ago we upgraded our truck to an extended cab, and our TC to a bigger, fully self contained unit. We started planning camping trips with our grandkids, who were ages 3 and 6 at the time. They had been used to spending time in our small Sunlite TC when visiting , having lunches, playing games and pretend camping by taking naps in the cabover.
The first couple of times we went "actual" camping, they slept with Grammy in the cabover and I was designated "chopped liver" and got the dinette. Then we had them start using the dinette and I graduated up to the big people spot in the cabover. We never had any issues with them being scared or disoriented in the TC. We do leave a very dim lit (amber colored) led flashlites in the kitchen and bathroom areas .
As you commented, try a dry run in the parents driveway, be sure to have him help in the planning for the "campout", that will give him a feeling of some control of the situation.
Some of the best times we have with our grandkids is when we are planning our upcoming trips. Keep it simple, don't overthink it, and all of you will have a great time. You will also find that one trip won't be enough for your nephew. Many more are in your future.
I do not see you as being paranoid at all. I honestly commend you on your concern. I do remember very well the first time we took a seven year old camping. It was a nightmare for us as well as the granddaughter. She cried all night, wanting her mommy. I am sure it was very traumatic for her. We should have spent the first night in front of their garage. She is older now, and no problem.
The wife and I have two children, both in their forties, and grand kids from 7 to 28 years old. Not a single one of them had instructions tattooed on their butts, and each were/are very different.
If you don't want the 160 decibel alarm. Something quieter, but not a regular night time sound....
-- Shop Keeper's Bell attached to the door.
-- mechanical entry chime - no power needed. As the door opens, the magnet pulls away from the door frame, causing the magnet to hit the chime bar which makes a sound.
-- mini wind chimes - If there is a local craft store, see if they have mini aluminum wind chimes you can string/thread along the door knob that will chime when banged about.
You obviously bought the wrong Truck Camper. You should have bought a LANCE, grown adults can't figure out how to open my door, to get out. I have seen a few people panic, trying to get that thing opened.
It is good to have concerns, that is what parenting is all about. The best answer was earlier, visit the parents with your concern/s and if this kid has lived to be 4 years old, they will have an answer for you. I get the idea that you have not raised any kids and your concerns make me think you are going to be a good Uncle, because you do have concerns. You get points for willing to do this, with your nephew and making memories together. My kids were backpacking before they were 4 years old and both are still alive and thriving. This young man is going to be fine.
Oh, easy alarm? Get a length of clothes line, tape/or tie it to a door and the other end tied to a few pots and pans, set on the edge of the counter.
Sleep tight, don't let the bed bugs bite.
Ps. Are you ready when he gets up in the middle of the night and needs to go Potty? I will wait for that next thread from you.