Personally, I'd think that, while it might be cool for them to have more stuff to hand out, it's also largely for the benefit of the people they're going to see. And while a lot of people might be like, "Hey, I'd love to do this if I could afford to," it sounds like they can afford to, so for $10 or $20 a person who can't currently afford to go over can at least do something towards it. I don't think you're in any way required to, but I don't think it's off for them to let people know that they're headed over there and that people who want to help but can't make it over, can help in this way. But you're not obligated, and in fact, if you'd rather save that money towards a trip yourself once your kids are old enough, then that's just as good.
If you do decide to donate, consider school supplies. I've heard that paper, notebooks, and writing supplies are particularly useful.