Sheesh, you all need to fuck off! YABU (so am I by my vitriol, but I don't care!). Honestly, ever thought that the reason they ask for vouchers is so their kid can actually (save to) get something they really do want rather than the umpteenth gift (cheap or expensive) that will sit in a corner gathering dust? Just shows how bloody loony some of you are - you care more about your fucking imagined 'principles' than respecting the fact that a parent knows best and is well within their rights to tell you what their child wants/needs. If you feel so strongly about it, don't take a gift at all!
I have, on numerous occasions, told people not to bring presents at all for my DC's birthday party, and their engaging company was all that was required. I have too little room in my house and didn't want to fill it further with stuff DC already had/didn't want or need. Obviously, some stubborn kind souls insisted on bringing along unwanted/unrequested pieces of plastic tat heaven - I smiled and graciously took it whilst inwardly thinking, FFS, I really want to dump it in the middle of your living room! Ungrateful? How can you be ungrateful to people who blatantly choose to disregard your entirely reasonable request? I figured out who was being rude and it certainly wasn't me.
And don't tell me I could have eBayed the items afterwards - I'd then be spending my time and money listing and trying to sell the offending tat well-thought out toy to the next mug person - an item I didn't want in the first place!
And breathe!
For kids' parties, I just give them money now - much rather it was used for something the celebrant/family wants or needs rather than what I think/believe they should have. Where I was born/brought up, I used to get money as a kid and remember the anticipation of putting aside money for a much wanted toy/book - it taught me the value of money and the concept of saving, and I STILL got a gift at the end. And thank-you cards were completely unheard of, but that's a whole other conversation ...