Bit of background: we live in a different country to BIL (DH's brother) and his wife. They have three DC aged from 5 to 9 who are our DC's only first cousins. DH has a longstanding arrangement with his DB that we will exchange presents for the kids at birthdays and Xmas, but nothing for adults - suits us all fine.
We have to post the packages and we try to post them in plenty of time so they don't arrive too late. It's become clear to us recently that SIL has been letting the kids open their presents as soon as they arrive, well before their actual birthday or Christmas. DH mentioned this to his DB a few months ago and BIL seemed pretty embarrassed and said it was SIL's decision to let them do it.
Now it's clearly up to them what parenting decisions they make generally, and they certainly do a lot of things very differently to us. I imagine they (or at least she) disapprove of certain things we do as well! But letting them open the presents immediately seems to be rather disrepectful of the effort we have gone to in choosing and sending them. We were both brought up with the idea that children should learn to wait for certain treats or privileges rather than thinking they can rip open a present the moment it arrives. We're certainly not fuming about it - it's a relatively small matter in absolute terms, but it leaves us with the feeling of 'why should we bother with nice Xmas prezzies?'.
So, would/do you let your DC open presents immediately if they are meant for another purpose, or would you feel that the present you've sent is being dissed if the recipient doesn't wait.
In the interest of full disclosure: there's always been a bit of tension between SIL and DH/myself in the 15 years since we've known her. We've tried to keep it under wraps for the sake of the children, but I'm not sure here if our feelings about the presents are connected to the dislike we already have for her, or if it's genuinely disrespectful behaviour on her part to open presents straight away.