Congrats to you and yours, Hattie! I don't have any experience with adoption, but think PPs gave you great explanations about the idea of telling the truth so early. It makes perfect sense to me now and I'm glad you see that, too.
In the end, though, it's up to the parents to decide how to move forward. So even if you still thought it was too soon, if they decided to tell the child about it now, you would be unwise to try to stop them, I believe. I know you want what's best for baby but so do they. Also, I kind of hate to admit it, but often it seems young parents will take professional advice over ours, anyway.
Please just enjoy your new GS and let DD (dear daughter) and SIL make the childrearing decisions (but perhaps, you generally do).