You must know which agency was dealing with your niece as someone from there contacted you about the possibility of your taking the child.
So you can contact that agency and ask if you can leave a letter on file for your niece, which she will be able to access if she wants when she is an adult.
You might be able to put money in trust for her now and leave the details of your solicitor in that letter. Then if she wishes she can contact your solicitors and get the bequest when she is an adult.
But you would need to think of the best age for her to access the trust, it’s probably not 18.
I don’t think it would work for you to leave money in your will. Because you don’t know when you will die and how would your solicitor contact her then ? Social services will probably have no contact with her once the adoption order is granted, she and her family could move away the next week. And you won’t know her name to trace her.
You could try to get In touch through an intermediary when she’s 18. But you are wrong to think that they will Trace her for you, they will contact HER to see if she wants to have contact with you. She might well not reply or just say no.
And if you are dead, how exactly will you use the intermediary?
I have to warn you that very few 18 year olds, adopted or not, have the slightest interest in their family tree . It’s normal for them to be focussed on the present and their own future, not ancient history.
I know can be upsetting, but adolescence is about loosening family ties, not making new ones.