keeping a playgroup together

My 3-year-old daughter and I are in a wonderful playgroup that we've
been attending since her birth. We've been meeting one weekday morning
a week, plus having once-a-month "mom's night out" dinners.

Most of the group is stay-at-home moms, with several of us working part
time. For the past 3 years, we've all been arranging our schedules
around this group because we really want to stick together. Some of use
schedule our work schedules around it, we don't take certain classes
because of the conflict, and so on.

Next fall, nearly all the children are starting preschool. Some will go
Mon/Wed/Fri and some Tues/Thurs, which means there is no day we can
continue meeting when most people can come.

Have you been in a playgroup like this that survived preschool and
stayed in close touch? Any tips on how to do this?

We're spread over a fairly large geographical area, so we don't
generally all run into each other as part of other community events or
organizations.

We've considered:
-- we will definately will keep having the mom's night outs, but we
don't think that will be enough to maintain the cohesiveness we have
now.
-- We might have 2 playdates a week, one for each sub-group. We worry
that this will deteriorate -- if we have a bunch of weeks where only 2
or 3 people show up, it will probably fall apart.
-- We talked about meeting in the afternoons, but most of the
3-year-old nap then. Many people have younger kids too, who will be
napping for years to come.
-- meet occasionally on the weekend. Again, afraid attendance will be
low.

Any experience? We have about 12 kids in the group. The weekly
playgroups usually have 3-9 kids each time now.

Thanks!

Jan