Hi, as a Birth Mother can I answer from the other side?
My daughter went to live with her Adoptive Parents when she was 18 months old. She is now almost 5, and though letter-box contact was agreed from the beginning, I only got my first letter last year! I haven't had one yet this year, and am not totally sure if I am getting one. I feel like they only wrote last year because we kept asking the letter-box co-ordinator about it, and I did eventually get a letter. Writing one back was difficult. I didn't know what to say!
I do know, however, that is up to the Adoptive Parents whether they want to do letter-box contact or not. Even if it something social services agree in court, it's not a legal requirement, and I don't think you can get any sort of Contact Order. It is also up to the Adoptive Parents when letter-box contact starts. There are many reasons why Adoptive Parents don't start it straight away. Maybe the child is unsettled and needs more time to settle in. Maybe they arrived and have to start a new school as well as settle in. Maybe they are unwell for a while. Sometimes the Adoptive Parents decide that they're not ready to start it, because they feel it wouldn't be beneficial to the child - like the child has been in Care a while, and had a lot of contact with their Birth Parents and they need some time to understand why they can no longer see them, and it would make them more unsettled to write immediately. Sometimes an older child - who understands what is happening - doesn't want contact with their Birth Parents. And sometimes the Adoptive Parents may decide that, for now, it's not in the child's best interest or they're too young to understand. In my letter, for example, they put that they were writing on behalf of my daughter - who was too young to write herself and too young to understand - and signed just their names on the bottom. My letter back had to be to them, rather than to my daughter, as they feel that at the moment she's too young to understand anything.
What I guess I'm saying is, don't feel too much pressure to begin. It's up to you, as the Adoptive Parent, to decide what is best for your child. That is something that the Birth Parents have to respect. If you feel you or your child isn't ready for it, then you need to let Social Services know. Remember, that your child needs to understand what is happening, and will need plenty of support in understanding and dealing with their feelings when they get a letter back from their Birth Parents. Ultimately, it's the Adoptive Parent's decision.
As for what to write, the letter-box co-ordinator can help with that, but usually the Adoptive Parents write things about what activities they like or what they like to eat (as an example of what my dughter's Adoptive Parents told me - my daughter loves spaghetti, playing with play-doh, art and craft, football, ballet and music). It's recommended that the letter is no longer than one A4 piece of paper. If the child is a little older, sometimes they include a drawing or they might write their own name on the bottom, but again, this is up to you.
Also, a lot of Birth Parents appreciate it if the letter is typed, rather than hand written, especially if they have Learning Disabilities or are Dyslexic. I think it's just easier to read.
Remember that the letter is read by the letter-box co-ordinator before it's sent to the Birth Parents, so if there is anything in it that they don't think should be in there, they will let you know and either ask you to re-write it (which I guess is why it's easier to have it typed, that way you can just edit it! - Oh, and also if you type it, then you can save it on your computer so you have a copy for your child!) or they will edit it out themselves.
I hope any of this helps?
Good luck!