(A few years back for me but) I rang the health visitor who had a "sleep team service" so we had an experienced health visitor who came to the house every week or so until better sleep habits established. From memory, she asked us to keep a sleep diary: what you did before bedtime, what time you put them down, what you did to settle them, what time they got to sleep, what time they woke in night, what you did, when they went back to sleep. Etc. And the same for how much sleep you got.
What helped was a better bedtime routine at a regular time, bath, supper, teeth, a long story cuddled up in bed. As calm as possible and low light.
I think we also started sitting on the bed while they went to sleep, then on the floor next to the bed, then a foot away, etc until you're in the doorway then outside room then at top of stairs etc.
What also helped a lot was a sticker chart altough mine were a bit older, about 3 1/2 and nearly 2. Might be worth a try though. Make it achievable, explain how they're getting grown up and learning to sleep in their own bed and get sticker for not getting out of bed while going to sleep. Print out a nice looking chart and give them lots of praise when they earn a sticker. If get five stickers they get a present (comic or something). When that well established eventually give stickers for going all night. We over promised on a big gift for this last bit not realising that it was a set that cost nearly £100. I was a bit
when looked at the price but followed through and bought it and to be honest it would have been worth £1000
Don't know if the health visitors still do this but worth asking. I was desperate when I rang as had two no sleepers that thought bedtime was fun time. And would wake in the night and only go to sleep if DH or I were with them. I would fall asleep knelt on the floor next to their bed I was so tired. How I never crashed the car I'll never know.