It's always a worry for parents. "I've spent all this money on a photo shoot, what if they don't behave?!" "What if they're silly in front of the camera and we don't get any nice images?" My advice... don't worry about it. As a mum of two, I understand. I'm not getting impatient and I'm definitely not judging. Kids will be kids and it happens.
Before Christmas I had the pleasure of meeting Freddie and Scarlett for a photo shoot in #TowneleyPark #Lancashire. Although Scarlett was a natural, Freddie was feeling a little under the weather and really wasn't bothered for the camera. He was however the most gentle big brother and really looked after his little sister. I could see he felt comfortable with Scarlett so instead of persuading him to do things he didn't want to, I focused on his relationship with his sister. He had some cuddles with Mummy during which I focused on Scarlett. His individual portraits were taken naturally whilst he was taking me on a little adventure through the woodland, he didn't even notice the camera during this time.
My point is that the you really shouldn't be worrying about whether the kids will behave, but rather asking, can my photographer deal with it if they do? Are they experienced enough to know how to deal with uncooperative children? If the answer is yes then you need not worry.