I don't think anyone can say I am not Pro-PTO, but I also know that whereas a school is an absolute necessity, a PTO/PTA is not. It is a luxury for the school.
I've got to go with LauriB on this. I have been in the same boat, where I didn't think it was a big deal to have my 8 year old with me in the back, after hours, while I was working on PTO stuff. Thing is, if my child gets to be in the back, why can't all the kids come in the back? One, it is school policy not to have children in the back. Two, it is, after all, the teacher's lounge - a private domain...lol and Three, there is someone in the front office who will keep an eye on my child while I finish up. If this is a real problem for you, then you need to either go home and do the work, or get more help earlier in the day so that the work is done before the final bell. I know it seems harsh, but I don't think the principal is being unreasonable. It would be wonderful if we all had principals like oneoldmom's, but hers is more the exception than the norm. Remember that the school is the principal's domain and you are just (ewwwww, I hate this sentence) there to help. You have to go along with the rules they set up. They are not there to cater to your needs, you are there to help theirs.
I sympathize and empathize, but the principal has the final say on this issue! If you and the principal can come to an agreement, great! Otherwise, you just have to go with the flow (and the principal...lol) on this one.
Good Luck!