Blimey, so many responses, wasn't expecting that. Thanks for them, and (for the most part!) I genuinely don't mind the more challenging ones at all. I know what AIBU is like, and actually I asked here, rather than say parenting, because I wanted that level of honesty. I didn't want it sugar coated because I was genuinely not sure if it was a good idea or not.
In answer to some points: almost all the kids asked go to the same preschool. It isn't my favourite charity, it's one all the parents already support, which is what made me think of it. I wouldn't do this for a pet cause without it being one all the parents (bar 4 families) are also affiliated with. I'd not think that appropriate, no. It's effectively a preschool party, anyway, but on reflection I think I will just leave off the idea of any fundraising there, because it's not fair if it makes some people uncomfortable, which the comments here make it clear it will. Captive audience, effectively, so that's a no go.
The other thing I think people are missing is that this isn't "fundraising at the expense of the boys", because the other family raised not wanting presents without any question of fundraising, weeks ago. I only thought of asking for donations instead last night, and then thought I'd canvass opinions here before taking the idea any further. I'm glad I have, as the opinions expressed have decided me against it. I'm far less certain that it's a good idea for a not-quite-4 year old to have 30 individual presents from one party, frankly. The two issues are separate, and always were.
Quint, you say you think it's rude to discuss friends' financial situation. I would think telling them to STFU about it when they choose to raise it is rude, personally. Our ideas of what is and isn't appropriate are plainly very different. If you are assuming that the other mother and I have been gossiping about that, then your assumption is quite simply wrong. And extremely rude in its own right. Saying "yeah, and things are tough for some people" as an addendum to not wanting the kids to be overwhelmed/spoilt is not sharing confidences. As to thinking cheap presents aren't good enough - I'd be bloody horrified if people spent a lot of money on a preschooler's gift unless they were a close relative or a godparent. What kind of presents should I be assuming people will buy?
I also think people talking about teaching their kids about the pleasure of giving are right, but not about stuff bought from a shop at the age of 3. We do that with DS over older people's presents, absolutely, as we generally make them, but not for gifts for contemporaries as I don't think most little kids would really value a handmade drawing or model done by another toddler. It's also not unnatural for a very small child to want a present chosen for their peer for themselves, and there are plenty of other opportunities to encourage generosity and empathy that don't involve attaching the display of those qualities to material possessions, IMO. Such as asking all the kids at preschool (while we can rely on guests's parents remaining and making the supervision feasible), rather than getting into the "you're not my friend and you can't come to my party" nonsense a lot of them are already starting. I don't really feel comfortable with linking stuff and love in their minds any more than it is already. At the risk of sounding like a hippy, kids have way, way more than we ever did in the 70s and 80s, and I don't think that an improvement in their quality of life. We regularly do sweeps and remove stuff he doesn't play with for a week or two, and if he doesn't notice, rehome it - he rarely clicks, and isn't bothered if he does. To me, that's the sure sign of a kid with too much stuff. (And please don't anyone come back and tell me to dictate to my inlaws how to do it, because I am not going to go there. I value good family relations too much.) Nor do I think the correlation between personal value as an individual and what you can afford to buy is a healthy one. That doesn't mean I deny my kid toys etc, but it does mean I don't want to teach him that giving people something you can just walk into a shop and buy shows love. It doesn't. Treating them well does that.
I really do like the idea of getting the kids to make a gift, but TBH that seems like a big imposition on the parents. But I think if we get finger paints, a giant scrapbook for each birthday boy, and get the guests to do a handprint each in it, that would be a really nice momento in years ahead. Won't mean a thing now, but I would love something like that from my own childhood parties now. So thanks for those who suggested something personalised.
Worra I think your logic is deficient, TBH. You think it's mean to deprive a kid of 30 presents, but that a big active party with all their friends could be binned in favour for having 3 over for tea. He has 3 friends over for tea just about every week. That's not a party, and I would think that really was depriving a kid of part of the fun of childhood. Considering how much the hall hire, bouncy castle, food, drinks, entertainment is going to cost...the OP could have donated all that money to the school and just had a birthday tea for her child instead. Hall is £30. Castle is £50. Food we are making ourselves and would have to make where-ever the party was; ditto drinks. All those costs are being split between two families, too. It's going to be a grand total of about £60 or £70 quid each. And what makes you think I don't do other fundraising stuiff for them, anyway? As for entertainment... I don't think Musical Bumps is terribly expensive, really. And again, we'd be doing that kind of thing wherever we were (though given our houses are small, we'd have trouble). 
This party will cost less than one at the local soft play centre, and it will be for three times as many kids.
Someone asked if I've ever asked for donations instead of gifts for my own birthdays. Answer is yes.
I do wonder if the people certain the boys'd notice and mind lack of presents, though, have 3/4 year olds, without any older siblings? Because I honest to God don't think they would know the difference at this age. Their expectations are moulded by those around them, and we've not been to a single party where presents were opened in front of the guests. Not one. Every single party DS has ever been to, parents hand pressies at the door, they go in a bag, kids don't really think again about them. DS is very excited about his party and has talked about it endlessly, but presents hasn't been mentioned in that connection once. (Unlike birthday presents from family, which he is well aware of). He's 3, is that unusual? I wouldn't deny presents if I thought he would notice or care, but I don't think he would this year. Should probably say that my own idea on this is likely skewed because my Mum had one big party for us all at the local common every summer, no individual ones (cost reasons) and we never got presents. I didn't even twig that we hadn't until people started commenting on this thread, tbh. So the whole "parties being about presents" isn't something I really think of. Parties to me were always about games and food, not loot. That may be why the presents thing just isn't an issue to me (and DS had a party last year, not on or for his birthday, so perhaps that's why he doesn't associate them with presents yet either?), but then again, I wasn't the parent to raise the idea of no gifts.
One person here has suggested that we ask that only one present is bought per guest, and then we split them, which I think is a good one and worth exploring. I'll maybe mention that as an alternative, given so many think they'd just bring a present anyway which rather defeats the object.
Thanks for the views - they've been really varied and I am genuinely appreciative of the overwhelming majority, whatever the opinion expressed, as I'd not really thought it through in any detail and so having my thoughts clarified like this has been helpful. As donations would make several uncomfortable, I definitely won't be doing that, and I will have a think and a chat about how to phrase it if we don't want presents, or if we want the presents to be split between the boys rather than each.
Sorry this is so long. There were just a lot of comments to respond to. Again, thanks for all the input.