4yr old sleep issues- Advice please!

[lingmerth]

My 4 yr old dgd has never been a good sleeper. However before Xmas she was finally sleeping through the night 4-5 times a week.
Just before she went back to school nursery after the Xmas hols she discovered she could get out of bed and go into her parents bedroom!
Since then she's done it every night varying times, earliest 10.30 to latest 2.30am. She won't sleep in their bed so my daughter takes her back to her bed and gets in with her. Dgd goes back off to sleep within half an hour but it's never a deep sleep and half wakes every so often saying 'mummy' to make sure mummy is still there. She then wakes between 6-6.30am.
My dd is shattered. Dgd doesn't seem to need much sleep and is on the go all day.
Is this just a stage, is it separation anxiety? Anyone in a similar position?
Thanks for reading to the end!

Does she have a gro clock? I would use that and set it to the later time like 2.30, tell her that's when she can get out of bed, BUT after a week of her being used to it start to creep the time forward every few nights.
My DS was always awake in the night at random times, usually asking for a drink and a snack, though he'd rarely get it. All stopped completely when he started school as he was so tired. Solid 12 hours every night then.