To expect a thank you for birthday money in the post?

[biethdayquestion]

I live 3 hours away from db and sil so whenever it's their dc birthday and I send a card with money I never hear anything so I'm often wondering if they got it.
Dm lives near by and always tells me (when I ask) they got it because she saw it in the mantle piece but they wouldn't know she tell me this.
Is it not the norm now to say thank you anymore? They're very popular and social people, but due to the distance between us we only manage to see them a couple of times a year when we're down and to be fair they do host a nice day and go all out bbq and drinks wise.
I just find it weird I'm always wondering if nieces and nephews received their money? Some are old enough to text, some are smaller just wondering what your thoughts are?

I remember getting an expensive wedding present for ex dh brother when he got married. It was in USA so I gave it to his mother to bring over. Never heard a word about it. I eventually asked his mother did they get it and she looked at me like
I had 3 heads. I guess so she said. So rude imo. Hate lack of manners .

OP YANBU. It’s basic manners and a really important lesson to instil in children that we take the time to thank someone for their thoughts and generosity. However I’ll be honest it seems less and less common these days.