Try this: www.laparks.org/dos/sports/soccer.htm

look into any local YMCA

You sing up for town or city soccer club.

Look for a recreational league put together by your town or community. You can google recreational soccer and the city you live in. That will get you places to start. This will allow you to see if they like it and they are going to stay with it or not, before paying for them to move up to the competitive level.

I would start at a Recreation department close by first. Scott has the right idea.
That way you want have much money involved if they decide they don't like it.

start with intramurals or if they are old enough for thrit school, if they grow to like it support them playing more vompetitively, sports are an important part of a child's life