All I have to say is "ugh!" Tacky.
Little children & old need to sit down with their parents as an "event" and write thank you cards for any & all gifts they receive -- birthday; Christmas -- you name it. We are raising a generation of "ingrates."
Seriously, I don't think I am too harsh on this appraisal. We have children who expect a lot out of life without giving forth too much effort. If you want to have a big party with lots & lots of friends, you need to take the time after the party to reflect and write a thank you card for each individual who took the time & thought so much of you to attend your party.
When we try to expedite or rush through social customs, it feels exactly as your gut reaction told you "tacky." I do not want to spend the time to address each note, so here's your thank you card.
The nicest cards I have received were individualized messages, but that's going above & beyond the call of duty now days.
About opening gifts at parties -- in the north -- it isn't the custom. Sometimes it can turn the party into a "materialistic" frenzy with the children wanting a present they see or wanting to open or play with the birthday child's present. Plus, it is BORING to children (and adults except for maybe grandparents who buy the bulk of the gifts).
However, if gifts were open at a later date (as when brides & grooms marry), and a specific thank you card is sent -- I think that takes care of one wanting to know how the child likes the gift and so forth.
But I've been to parties when the gifts were open & when the gifts weren't...........boring or not -- the hostess rules & it is another way to show public gratitude -- so I can understand it.
I do think the microwave thank you notes are exactly what they seem like -- hurried & unthoughtful.....but better than no thank you card.