You could get a
triple + a 2-seat trailer for the little ones. The folder triple will grow w/ the kids to adulthood.
Or a
tandem trail-a-bike plus a 2-seat trailer and pull them
all behind your one-seat bike. That seems like a l-o-o-ng train to be pulling to me.
Or, a
big-dummy/xtracycle could probably sit 2 big kids on the snapdeck and pull a 2-seat trailer behind it.
(not sure I added anything but
color to masiman's post but it's fun to look through those sites...)
ps-> once my older outgrew the trailer we have been kinda stuck on how to move just 2 kids at once, haven't taken any steps beyond yet.