I should add that my son is 18 months old, and for the most part, still pre-verbal. So, if I simply told him to stay in bec I don't think he would understand.
we made a biggg deal out of it- new sheets-let them pick out a night lights let them go round the room and pick out their favorite loveys putting them into a bag- set them all round him at night time and tell him to help his sweeties go to sleep-- he can kiss them all "nite nite"
nonverbal is ok- because you are training him- he doesn't have to really understand- like he trained to hold his own bottle- or that when he babbled, it made you smile! And when you said no no- you mean it!- He didn't have to understand- you see?
the first few nights may (WILL) be hardest- never get emotional- but show that you will win- say "nite nite"-or whatever words you use--- first time he gets out of bed you say nite nite- but after that with zero emotion- and say NOTHING- lovingly walk them back to bed and put them in bed- it may be exhausting- and yes it may take hours- (sorry) do not lose your temper and do not give up or you are compromising the whole thing- and the whole time you put into it was just torturing him, if you do give up!..... do it on a Friday night or other night when you all can sleep late-
the other hard parts for me was when they were sick- I let them in bed with me- compromising everything once again.
I do not envy you but pray it all goes well right off the bat-