As Ellen explained it to me, it sounds like this group sees the bday parties as a more elaborate extension of the twice monthly get togethers and have come to expect that everyone will take a turn hosting one each year. She did mention that so far at least one family hosted a joint bday party for their two kids, which ended up not being during either child's birth month. So I guess it's good a precendent hasn't been set that each family hosts one for every kids separately.
That's kind of where I was going when I read this. I'd actually go as far as to suggest that Ellen's actually 'doing it wrong' (in air quotes) with their individual invitations to the hosts. Look at it this way; if there are eight families and sixteen kids (they'll there soon, I expect) doing this, each family has:
* Two birthday parties to host
* Fourteen 'thank you' dinners to attend with their guests from these parties
* Fourteen birthday parties to attend
* Fourteen 'thank you' dinners to host for the parties they attend.
Way too much for one year, especially in addition to their twice-monthly activities!
That said, if the group norm is to use birthdays to rotate hosting, they do have an obligation to host. But they can try things their own way:
* Host a summer picnic at a park unrelated to a birthday.
* Have everyone over for a birthday pancake breakfast.
* Organize a simple birthday playdate for the kids that's not an actual party and just provide light snacks for any parents that come (our group does this often).
* Do something else that reflects their family's interests and values but considers the group dynamics.
You never know, they might start a shift in trend - also breaking up the boredom - without unsettling the group!