help us identify this caterpillar

[Chris Ruim]

Our back yard in southwestern British Columbia, Canada, has lots of
large, black caterpillars.  They have a row of yellow spots down each
side of their body and a red spike on their back end.  Our two year old
is fascinated by these, but we have no idea about a few things:  What
kind are they and what knid of butterfly/moth will they become?  That
spike looks a little scary, is it dangerous?  If we wanted to keep one
and see it turn into a butterfly in the spring, how would we go about
it?
Any help for these 'rookie' internet users would be very much
appreciated!
Thanks,
cruim at direct.ca