I cannot believe how sanctimonious everyone is being about this.
Basically, we all know that children's parties are hell on wheels. No - they're hell on e numbers - that you have to organise and then clear up after.
You do it and then ... relax ... and watch on with delight as dc enjoy all the other hell on e numbers that aren't, mercifully, your problem.
It's kind of like everyone putting something into the pot and then taking a bit out again.
These mums aren't playing in the spirit of the game. It's the Prisoner's Dilemma and these mums have gone for a cut-throat defence. They've cheated, taking advantage of the fortuitous stroke of luck of coincidental brthdays.
You're all way to hung-up on the present thing. what they've cheated on is the suffering.
It's my belief that when your dcs are getting bigger and your thoughts start roaming towards the baby section of Gap, it's not the idea of dimly remembered sleepless nights that puts you off embarking on the joys of buggy-buying, it's the immediate and present horror of the birthday party that does the trick.
And where will it all end? the Prisoner's dilemma illustrates that it is ultimately in the long-term interests of all to co-operate and behave altruistically. I foresee a trend of conglomerate birthdaying, where parents group together in mass birthday parties and those lone souls, with dcs born in September glumly putting on parties all by themselves.
You don't want that, do you? Surely?