This sounds so hard, and you sound like a fantastic mum Rose. Some of the replies here display some very rigid, black and white thinking which is really unhelpful, not to mention accusatory and hurtful.
I didn’t sleep well as a child due to anxiety, but in my case I was older, having slept reasonably well as a very small child, as far as I’m aware. I did also have some physical health problems as a child which may or may not have had an effect on my sleep.
The anxiety could be about anything. When I was younger, giants and monsters were the focus of my fear. As I got older, it centred on fears about my parents dying; worrying that I was adopted and that my parents were keeping it a secret from me; worrying that I might be the only person in the world who was awake. Anything really. This went on till I was about 10 or 11, intermittently. I remember my parents putting a mattress on the floor of their bedroom and telling me I could go there if I needed to in the night, which I did now and then.
I realise this is probably not helpful to you OP, as I am only sharing an anecdote and have no advice, but just want to say that that phase of my childhood was difficult, and I suspect it would have been a lot more difficult if my parents had not responded with loving kindness as they did.
I felt so, so, so alone at night. I think that was the crux of it. I was quite a sensitive child. I’m grateful that my parents didn’t force me to be alone on those nights.
And for what it’s worth, I’ve turned out to be a reasonably normal adult. I can even sleep all night long in my own bed without needing any snowflake treatment 😉