Do you know, I think EVERY mother worries about this before a photoshoot! Even me before our last family session. Totally normal. Totally not even an issue. I have 2 kids and I've been around enough other children that nothing phases me now, kids will be kids. I like to think that I have a pretty endless supply of patience when it comes to other peoples children!
At the start of the session I always get down to their level and find out how they are feeling about what's about to happen. It can be really scary or daunting to walk into a studio with strange things and meet this strange lady. Some kids take a while to warm up and come out of their shell, others are full of life for 30mins straight. That's all ok! Then the session just evolves/moves according to how they feel.
I make it as fun as possible and I also get the parents involved right from the start, that way the children actually forget I'm there and we get those beautiful, genuine expressions.
I always try to plan the session time to work with optimal kid friendliness, like we might not schedule it for 5pm after a full day at preschool when everyone is hungry and tired!
One thing I always tell parents is yes, it's great to get that "everyone smiling at the camera" photo, but we also want to capture their unique personalities and expressions, because that's what makes them them right at that moment of time.