I have done sleepers with my three when they were smaller, and they loved it. We took them to the lounge car in their PJs for hot chocolate before bedtime, and they fell asleep pretty quickly.
It is a pretty late night for them, and we decided to let them stay up until the train left - but that did mean they were tired, and did drop off quickly.
We had them in interconnected cabins - it will depend on whether you are going first class or standard, and how many of you there are - but each pair of cabins has a connecting door, and with 5 of us, we had me in one cabin and ds1 in the interconnecting one, then dh in another cabin with both ds2 and ds3 in his interconnecting cabin.
There are nightlights in each cabin, so they will not be entirely in the dark.
The steam train from Fort William to Mallaig is great fun too. Though the time we did it, dh was not in the best of moods, and the train was busy so we ended up split up. I was in a pair of seats, with whichever of the dses was behaving worst, and periodically dh would come down the carriage and swap whichever child I had for whichever one was winding him up. I think he was not best pleased that they all behaved reasonably well for me.
Oh wow. I didn't think of taking him to the train lounge for hot chocolate. I was thinking pyjamas on at Euston station, and a hot chocolate at the station but going in the lounge sounds good. Definitely do that on the way back as the train leaves earlier.
As far as I can work out, we could have two standard carriages with interconnecting doors and bunk beds in each, if we pay the single supplement, as there will be three of us. There's not much difference between first and standard class, by the looks of things.
There is no difference at all, apart from the fact that the upper bunk is folded away, if the cabin is a First Class one, and out if it is Standard class. And you get a different breakfast - continental in Standard class, and a choice of continental, Hot Meat and Hot vegetarian in First class (if I recall correctly, and things haven't changed since I was last on a sleeper).
I suspect your dc will love it - mine found it a real adventure (as I do).
I was looking at this yesterday and decided sleeper to Edinburgh would be better as more to do when you get there. Looks like the steam train at Fort William stops on 25th Oct (maybe we have a different half term to everyone else??)
The reason I've picked Fort William is we can board the train at London at 8.30pm and arrives at FW at 10am next day. DS is five and it would be a nightmare getting the Edinburgh (or Cornwall) train at midnight.... There will be more hours for him to enjoy being on a sleeper train...
I was looking at going next Easter, but the steam train doesn't run then. (ARGH!) So might have a rethink on date or just go on the cheaper normal train to Mallaig over the Harry Potter viaduct.
One of the Grandma's at school is Scottish originally and went to FW last year to walk up Ben Nevis. She said Fort William is not a friendly place and said not to stay on any of the touristy hotels - stay in the Premier Inn!
I didn't think the Nevis Range centre was too far? 7 miles from Fort William? One of the trip advisor reviews said it was £10 for a cab there, and they got a bus back. Buses are every two hours.
Another option for a day trip that DS might like is catching the coach to Loch Ness. He's never been on a coach in the UK, he says, and knows about the Loch Ness Monster from Scooby Doo.