I do the sleeper all the time and it's fine . Not ideal in your situation but sounds like you don't have much choice .
You need two cabins adjacent to each other , both first class but one set up as second class . It's the same cabin, just set up differently. You might not be able to arrange this online, so get the the station early, at least an hour before departure, and the crew can arranged it for you . They are very obliging .
Dh takes your toddler in the second class, which has two bed and puts toddler in bottom bunk . Puts buggy on floor .
You go next door on your own. And take suitcase . It will be quite squashed as you are very Pg and have a large case ( I assume ) .
If by any chance you know someone who is travelling by car to the same wedding, I'd see if they could take a suitcase with wedding outfits/ gift for you .
It won't be comfortable for you as the beds are narrow, but you won't be sleeping well anyway at 36 weeks . Turn the temperate down low. Pull the mattress out a bit from the side to make the bed wider . Put the towel on the plastic pillow under the pillowcase .
The train gets in very early, so get to your hotel ASAP ( see if you can get an early check in ) and have a nap while Dh looks after toddler .
Take cereal for your toddlers breakfast , assuming that's what he normally has. Your get a breakfast on the first class sleeper but it's cooked or continental .
Do NOT I repeat NOT do sleeper both ways heavily PG unless you have a night or more in a hotel in between .
Other options are to take day train one way and sleeper the other . The 1630 train out of London takes only four hours to scotland . I'd pay the extra and go first class. Take electronic gadget and earphones for toddler. Do not book quiet coach unless your toddler is an angel or other passengers will hate you.
Tickets - you don't need a ticket for toddler. But it may be cheaper to get a family railcard for and buy him a ticket, as it means you get your adult tickets much cheaper . Avoid travelling on a Friday if you can (busy and expensive ) .
Book well in advance ( 3 months) to get the best deals. Check your dates are not on a Scottish public holiday .
Take your maternity notes !