Well, I don't know what the student population is like in the schools you are playing at, but in our school district, there are enough kids who have heard civil rights anthems, patriotic songs, etc. that you might want to include these as sing alongs the kids already know. About the only patriotic song I can tolerate is "America the Beautiful" which lends itself to some hilarious improv when you first (re)teach it to the kids (depending on the age group and student population), and then use the "Oh beautiful for..." line as the leaping off point for silly stuff.

I second Jim Mclean's suggestion of Woody's kids material. The family put out an excellent book/CD of it in the last decade or so, called "20 Grow Big Songs" or something like that.

I would disagree with the "Wee Sing" series. It always seemed a bit too dumbed down to me (please don't take that opinion as a personal comment on you Susan, as I certainly don't intend it as such). I don't think many inner city kids in racially integrated classrooms would know them anyway. Although if you are playing in the suburbs or a predominately white rural area, Susan could have a point and many kids in those areas might be familiar with them.

I don't think the kids need to be familiar with any of the music you perform though. If you are working in the schools, you are there to teach them about music, not act like a tape from home being played back to them. By all means, use anything and everything at your disposal to get them singing and learning new songs. Get them out of the nursery school comfort zone and into MUSIC. Don't dumb it down for them, but do make sure it is age appropriate for their developmental level.