17 month old is in long 2 piece pjs set & 4 tog duvet 5 year old is in long 2 piece pjs set & 7.5 tog duvet.
My LBs room is 19° and we're doing pj's and a 2.5tog sleeping bag. He's not too hot when he wakes. I put him in short sleeved pj's the other night and he was cold to touch when he woke in the night so I went back to long sleeved jammies x
Our room is around 19-20 and we just moved from 2.5 to 1.5 tog (she is in long pjs, no vest).
Im doing a short sleeve vest, footless pjs and 2.5 and the room is 17.5/18, so if I were you I would keep 2.5 and one layer of clothing. Or go to 1/1.5 tog and add a vest as well