“ It's always the same names on every thread” - you don’t often see my name on these threads, do you, VS?
I feel compelled to respond to your unpleasant insinuations, your value judgements of the OP: “You don't seem to know them well enough”…” The days of giving respect to get respect are here” (are you suggesting Agent isn’t respectful of her DGC?)…” Expecting a level of respect from children just for being an adult is wrong” (OP did not suggest this).
“I'm afraid the days of a stern telling off are gone”. Well my son and DiL wouldn’t agree with you.
“ I haven't been at all hard on the OP”. Oh, I think you have.
I’ve been in your shoes, OP. One never said thank you, the other disdainfully pushed an unopened present out of her way, overlooked it and left without it.
One year the former bemoaned others’ ingratitude at her generosity. When I joined in the conversation - the irony of the situation highly apparent to those around us - she became even more enthusiastic yet still the penny didn’t drop. Her mother silently and knowingly observed, having given up herself.
🤷♀️
Whilst I haven’t had this situation as a grandmother, the advice from PP to put £10 in a card seems wise.