Here in New York, we've had a blistering summer, with several heat waves of near-100 degree temps already, and it's only mid-July. We can only cringe to think what August has in store for us.
I'll take the heat over the cold any day, but when the humidity maxes out, things can get pretty nasty. Not the best scenario for shooting four kids and their mothers in a public park in White Plains, but certainly doable. As most parents know, kids are actually pretty resilient when it comes to extreme temperatures, so as a photographer, I'm usually more concerned with the parents (and myself) than the little ones.
Ideally, early morning or late afternoon, when cooler temps may prevail, is the best time for taking photographs of your kids. The light is much better, as well - those high noon shadows can wreak havoc on your photos. But children don't care about proper lighting, and nap times are more likely to dictate your scheduling than anything else.
If you're going for a photo shoot outside on a hot day, an area with a lot of shade is key. Although it makes lighting a bit trickier - the temps under a tree or gazebo like the one we had on this shoot, can be significantly cooler. Be sure to bring along lots of fluids - water is best, as you don't want juice stains on junior's white dress shirt before the portrait session gets underway. I usually bring a few extra bottles for the family I'm working with.
On the subject of dress, unless the shoot is part of a wedding or formal event, like a bat mitzvah, keep it casual. Not only will the kids be cooler, they're more likely to act naturally and have fun if they're in clothes they can easily move about in. And the best shots are often of them rolling around in the grass, getting messy. So yes, you might want to bring an extra change of clothes just in case.
Better yet, get those formal shots out of the way right off the bat. While on milder days, I prefer to have the kids loosen up and get comfortable with me and my camera, but on broiling afternoons, it may be best to get the group shots before their patience wears thin and their brows are dripping sweat. This may hold truer for parents, come to think of it.
Speaking of loosening up - what kid doesn't love bubbles? That's why I usually bring a couple of bottles to an outdoor shoot - it gets the kids excited and moving around, and most importantly, takes their focus off the camera and onto each other, which will get you much more relaxed and natural expressions in those candid shots.
For more tips on taking pictures of your kids, regardless of the weather, I highly recommend this "top ten" post on Adorama's website.
Oh, and don't forget the sunscreen...