I've done my photo session, and the advice on this thread was invaluable. I took three 24-exposure rolls in about an hour while chasing two kids around a park. I brought an army blanket to put on the ground for sitting poses, and mostly shot in the shade or in filtered sunlight. Then I put them on the play equipment for standing shots with their heads on the same level.
I had my sheet of white foamcore for a reflector, and I wore a lot of jewelry so I could give it to the little girl to play with while I took pictures. Her older brother was much more cooperative than she was, but is a stiff smiler, unfortunately. I got him unawares several times, so I have some better material than the "posed" shots.
I am going to do some things differently next time: use 400 ASA film rather than 200 when I am shooting fast-moving kids, and try some backlit situations as well as frontally lit ones. I also felt the lack of automatic focus, but that's going to have to wait until I get a new camera. Mom was rather in the way and issuing orders to the kids--"Smile! I told you to smile!" But I'm not sure how you get her out of the picture, so to speak. I made her hold the foamcore, though.
After looking at the developed rolls, about a dozen of the photos are possibles, and about six of those are good enough to show to the parents, though I certainly have a way to go before I'm proficient at this. I'm very glad I asked some questions first!