For volunteering, I've had a lot of fun working with EarthCorps. They do restoration and planting work at parks around Seattle, including many in the North End, and children can help out as long as they have a parent with them and sign a waiver. Here's a calendar of upcoming events that you can sign up for online:
http://www.earthcorps.org/volunteer.php
You could also volunteer at a local food bank. Most of them are always looking for help, and I'm sure some or most of them would welcome kids. I volunteered with my mom at the Family Works Food Bank in Wallingford when I was a kid and always had a good time. You can find a list of food banks in Seattle here: http://data.seattle.gov/dataset/Food-Banks/ryz5-i54h