I am a Producer and House Manager for a local theatre company that also has volunteer ushers. I have a couple of suggestions.
Politely, yet firmly ask them to please be quite, stop kicking, whatever. Don't shhhh, since that noise carries more loudly than whispered speaking. If they don't stop, definitely see an usher during intermission, but ask them to point you to the House Manager. Volunteer ushers are sometimes very experienced and can handle most anything, but sometimes not, so House Managers are your best bet for this sort of thing. Be as specific as you can about who is the problem, discretely pointing them out if possible. You should expect the HM to have a quick word with them about appropriate behavior. Usually, this does the trick. When we have these issues, I usually speak directly to the child (with the parent there of course), explaining the basic manners expected in a theater, and getting an agreement from the child that they will follow the rules. Usually, the parents are so embarrassed that their child needed 'the chat', that they are very good at enforcing those theater manners during the second half of the show.
As for Mr. I-know-nothing-about-history-but-plan-to-teach-you-and-everyone-else-anyway, I would just ask him politely, but firmly to be quiet. Don't worry about the accuracy of his commentary. That really isn't the concern, and will only cause a debate. The issue is talking during the performance, so just address that.
As for the late seating issue, please bring that to the attention of the HM during intermission as well. He/she will be glad for the information and use it to re-train the errant volunteers. :)
When I am in the audience at other theaters, I rarely hesitate to ask people to quiet down. I smile really big and say extra sweetly 'would you please be quiet.' I almost always get the desired result, and also a few 'thank yous' from those around me during intermission, because they were afraid to say anything.
Bravo for teaching your child basic manners within a theater!
Where were you? I know WVLO is currently doing Sound of Music, but that wouldn't be local to Pleasanton. Just curious! Also, I may know someone involved with that show and can ask them to pass the word of your experience to the HM. They can't fix a problem they are unaware of. :)