Regular poster with two small DC (both under 3), have NCd. Sorry for essay.
My DN is spending this term with my DM - DB runs his own business, DSIL works away during the week, and DN will go to school and probably get a bit more day to day attention than with DB, and didn't like her previous school either (and DB and DSIL agree it was very wrong for her, they live rurally and have little choice).
DN has never slept in her own room - I am pretty sure she was sharing a bed with my brother until about 8yo, then in an alcove off parents' room with no actual walls. Until she was too old to fall asleep on the sofa she did that, then just went to bed at the same time as DB. DB insists children do not need more sleep than adults (first hand from him) and DM thinks he was too lonely to stay up without her during the week when DSIL is often away overnight (secondhand from my DM). They have no form of routine at home anyway, never have (first hand observation).
Long story short, she is as per my DM a nightmare, and they've been to stay and I concur. She says she isn't tired and won't go to bed, keeps calling out from bed (she's been sharing with our older DC, who had a very disturbed night and was hell on wheels), leaving lights on ditto disturbing DC, insists she can sleep properly with light on in room and on landing, lies in bed reading or moaning for hours, and then frequently gets up and gets into bed with my DM who is not happy with this.
She does seem to be specifically anxious about going to bed and generally anxious (not surprisingly about new school, but she's always been an anxious child). Have suggested visit to GP.
DM sees my two excellent sleepers and wants advice!
I would tend to suggest some form of gradual retreat given age and anxiety (DM does end up reading outside her room for ages anyway) but am not sure how to structure this, or if there are any good alternatives for appalling sleepers at this older age.