First off, the clean up event that East Bay Regional Park District hosts on Martin Luther King, Jr Day at the MLK Jr Shoreline is really worth going to with kids. Well, it's worth going to regardless of your age. But there's something about it that works well for and with kids. I highly recommend it for you next year. We didn't register, we just showed up. Maybe my friends at East Bay Parks don't want me to say that, but it's what happened and it all worked out.
If you're going to take your kids to some kind of clean up or restoration event, first I say YES! And then I say, be sure to let them be kids. They don't need lectures or statistics or guilt trips. They will get it. They understand, at their age appropriate level, that finding one gold high heel or 17 empty plastic bottles in the pickleweed is not how things are supposed to happen in the natural world. And that knowledge and that experience will situate itself somewhere deep inside their being, and it will manifest itself later in life.
I'm proof of this.