Just put up a meetup notice. Watch out for needing to reserve a soccer field with the city, but for young kids, you can buy and bring your own small nets to any grassed area and have it work. School fields here are fair game on weekends when not in use. Bring lots of balls and those small cones. (each young kid needs a ball to have fun with drills and you add in short games).
I have set up a play games area in the park near a splash park, and we had a lot of kids join in randomly. (Ok, I had soccer balls and water guns... lots of water guns to share).
Parents are required to stay and watch, for kids under, say 8 or so.. or have someone (not you) responsible for their kids.... Have one parent walk the play area at the start, to look for any trash that could cause an injury (glass, piece of metal, broken something). Bring a good first aid kit for a slashed knee. (you never know when this will happen)