I think it's rude.

Maybe I'm old fashioned but this method shows no appreciation of what someone else has done. Is it so inconvenient to open a gift from grandma when it arrives? It doesn't need to be used right away. Kids at that age forget quickly. Set it aside and pull it out in a month and they'll get excited, all over again.

Being a mom of 2 littles plus living in small quarters, I prefer people ask what they need/want vs buying a random gift we'll struggle figuring out where to put.

I get many people may send gifts but out of courtesy to your elder family members, I feel it's respectful to consider their feelings before your own convenience. That's what having relationships mean.

I'm surprised your son was defensive. Maybe next time, ask him if there's a specific gift they're interested in, as the main gift, and then add whatever smaller add-on to the side.