Who sent us the letter from Chapter 1?

I don’t know if this has been answered already but I seen a comment on Tiktok basically wondering who sent us the letter that was at the start of chapter 1, (Find the poppy flower?) I mean poppy couldn’t have sent it right? But who else knew she was in there besides the prototype? I’m curious about this one!

Edit: And another thing to add edit, I seen someone ask “who were we?” And talk about this and I’m again curious…I mean mommy refers & knows us as the employee but like someone said how important were we? Important enough to receive to letter?