I'm really sorry to hear that the first letter was not good
Adoptive parents don't get a lot of guidance on what to write, some local authorities don't really give any. Your grandchilds adoptive parents might have had some general pointers but probably not a lot more than that. I remember being left to figure it out myself. I think lots of people struggle with how to write and present it and what tone to take. For my first letter to DD2's birth mum, I wanted to be chatty and friendly, and yet I was really worried that if I was too informal, it would come off wrong, like I wasn't taking it seriously, or something like that. Very difficult to write and I was still worried that I'd said the wrong thing
I think nowadays a lot of adopters are advised to write like it's a report, rather than a chatty letter. So like, write an A4 page with headings like "School", "Health" etc, and avoid any chatty paragraphs
I prefer to still write a chatty letter with the slightly increased risk of accidently saying something which causes offence, but I can see why some people struggle with that format
I think in your reply, try your best to be friendly and chatty, and give information on how you are all doing, any significant family news (births, deaths etc), any significant things you've done this year, if there's any information or anecdotes about your grandchilds early life that you think your grandchild might like to know about in future, include that, you can ask any questions you'd like an answer to about your grandchild
Who you are writing to, really should have been told to you. It's social services fault if they haven't told everyone what's happening.
If you don't know who you're writing to, I would say always write addressed to the adoptive parents, and I would write only to them, because it's usually the rule that letters don't go directly to the child, and so IMHO its safer to not do it. It's possible to my mind that SS have agreed with the adoptive parents that letters don't go to the child, and then forgotten to tell you that (which is stupid but that's SS for you)
However if you are up to it, I would contact SS and ask them exactly what the letterbox agreement is - who you're writing to etc.
You won't know whether your GC sees the letters, I'm afraid. Most adoptive parents don't show very young children letters and leave it until they are a bit older (at least school age, but often about 6-8) to talk about it. This is partly to avoid confusing a young child, but also because a lot of birth families write the first letter but then stop and never write again, and most adoptive families will therefore not talk to their child about it until they are reasonable sure that the letters aren't going to just stop. They don't want to tell their child and then have their child say in a years time "where's the letter" and it never arrive
I hope that's in some way helpful to you. I am really sorry letterbox has started in a bad way, and I really hope it gets better