Heya, [IMAGE'S LINK: https://flic.kr/p/ornGoz]IMG_9191
I'd stick to wider angles probably, like the 17-50. In a forest with trees and different foliage, it's really easy to have to stand back over 20 feet to use an 85, 70-200, 55-250, and get full body shots of two kids stick-jousting or something, let alone normal portraits of two kids, etc. Too much working distance. I find with kids, the closer you are, the better, without being uncomfortably close. I really like 35mm through 85mm for kids on full frame, or 22mm through 50mm on APS-C. You can still get nice soft backgrounds if you need it, without having to deal with the complexities of moving kids at long range with objects between you and them.
Taking one camera with the 85 F1.8 would be good for the "in the tree" shots or head shots, etc, full body at distance, especially in dark spots.
The other camera with the 17-50 for the general shots, probably shot at F4 when there's several kids. I'd rather crop out areas than lose an arm or something in a fun moment.
Definitely scope out the forest before the shoot. It may be a very dark place, to a camera which will have you pushing ISO pretty hard to keep a shutter that will stop their motion, and you probably don't want to shoot completely wide open with several subjects from a depth of field standpoint depending on what lens you use. A light can be nice to keep shadows out of their eyes. The important thing for exposure would be to expose up, so that you don't get dark faces, dark eyes, shooting in RAW, then drop exposure in post, but you can keep exposure on their faces without losing data or having to stretch data that way. Even if you don't use a light.
Scope out the location for bugs and stuff. Last thing you want is kids freaking out over spider webs, roaches, ants, etc. Plus mosquitoes and stuff really suck (get it!?).
If you're not already very comfortable with lighting, I would leave it at home honestly, it's a whole other world of hassle to corral children with a static light in the woods while asking them to play. I'd just go for candids. When they're tired, do the couples and group shots. And make sure to stop down plenty, worst thing is to have a kid out of depth of field in a group shot with their family.
If you have them in one spot, controlled, you can use lighting and use really shallow depth of field, but if they're running around, multiple of them in the shot at a time, it will be harder to do that.
I did this with a 5D classic & 85mm F1.8 @ F1.8 with a light (speedlite in a 24x24" softbox). See how dark the ambient was at 1/200, F1.8 and ISO 100? Forests are pretty dark! So if shooting at F4, that would put me at ISO 400 right away, and I'd probably want another stop of shutter to get closer to 1/400s, so ISO 800 to get something similar. And even then, if natural light, I'd want to expose a bit to the right to avoid under exposing a face/eyes as that's hard to recover sometimes, so I'd probably push go another +1/3rd of exposure from there, be it ISO or shutter speed, or a combination of the two.
It was a random pine forest on the side of the road.
Bugs were bad!
by Martin Wise
, on Flickr [IMAGE'S LINK: https://flic.kr/p/orn7fs]IMG_9202
by Martin Wise
, on Flickr