when we had ds2's bday party a few weeks ago we invited his nursery friends but a couple couldnt come. We got a lovely letter from one of the moms and apresent too, which was a book and i was a lovely surprise. Saying that though i wouldnt buy for a child who i didnt know and wasnt going to there party but i would send a card
If I'd already replied and said yes, but then later had to cancel and the party was held at a venue where the parent pays per child then yes I would definitely still buy a present.
If i had to reply no to an invitation straight off then no I wouldn't bother.
If it was my childs party I would find that OK.