If the wedding is child free becuase people actively do not wish to have children there then I guess it's a different kind of party from anything I would be wanting to go to. What I can't understand is making an actual edict for nobody to bring children "for financial reasons" - you can make it clear you will only be feeding the adults but allow people to bring children with them to be fed and watered at their own expense. Given the choice between "You must magic up from thin air someone to care for your child for £40" or "You can buy your children a meal for £40 each direct with the venue" then I'd take bringing her with us every time. If the venue will physically accommodate the children then it's a bit priggish. Some of our cousins have 9 kids, they know that they would never be invited anywhere if people had to cover all the expenses of their children.
It's not as if guests don't go to significant financial lengths also - bringing a present, travelling a long way, often getting accommodation, buying an outfit - they can be hundreds in the hole putting themselves out to bring themselves to a wedding. It's a bit different if you are inviting local people, I suppose, but then it's something I have never come across - I'd rather my family came and brought thousand of kids than leave out family to fit in a co-worker.
Then again, I've never gone to a wedding thinking "How nice, this will be lots of fun and I'll really enjoy it" it's a lot more "OMG I will have to drag myself to x & y's wedding, that's going to cost a fortune" so if they want to disrupt my life and give me mounds of expenses to come to their wedding but can I please not have any children because that's inconvenient, how is that not meant to be off? So maybe it depends on your relationship with everyone involved, and that's going to be different for each guest therefore the level of understanding each potential guest has is going to be variable.
At the end of the day, you can make the decisions but you can't control how people feel or tell them the way they feel is wrong. One can decide not to invite any children, but it's not reasonable to tell people they shouldn't be upset/offended about it - if they are, then they are!
Short answer, YANBU to feel that a child free wedding is perfectly OK, but YABU to think that everyone should agree with your viewpoint, YANBU to think they should conduct themselves with decorum even if they are offended!!