We made the kids write/draw thank you cards (they're 5 and 3) under threat we'd return the presents if they didn't. I think it's important.
It's polite, but in certain circles it's become as much a matter of pride for (often slightly paranoid) mums rather than genuine thanks from the kids. I know one (rather uptight) mum who gets her kids to send thank you for the thank you letter' letters... and I'm not kidding.
Yes, seriously!Seriously???????