Hi Annette!
A couple more thoughts for your shoot?
If you don’t already have it on there, add a 5in1 reflector/diffuser to your wish list. And to your external flash, add either radio triggers or a flash sync cable that is compatible with your Nikon gear (iTTL, I believe?). One example is
OCFGear.com. Be nice to have a shoot-through or reflective umbrella, or maybe a for-speedlight softbox. Also, you could bounce off the reflector or possibly shoot through the diffuser.
Press Sis-in-Law into assistantship to handle the reflector/diffuser/off-camera flash.
I would definitely use flash or at the very least reflected light of some kind. Or at the very least have it available. Especially if I were shooting subjects backlit by the sun. This to lift the shadows in the face. If its not incredibly bright outside you might even control the exposure of your ambient and bring out your subjects more.
If you can talk Boyfriend into an early birthday, acquire the gear and start working with it before the shoot.
Though you can't always control them completely, keep an eye on your backgrounds, watch for ‘branches-coming-out-of-heads”, things like that! If you can, shoot with your aperture open as much as possible to blur backgrounds.
Chances are that with the girls and the puppy, it’s going to be a fast-paced shoot! Especially chasing them around! At least until everyone gets worn down some! Blankets and ground pads are a great idea, but you may not be able to easily haul them around with you as you chase the shot. So just wear something that you are not afraid to get down and dirty in just in case.
Most of all, and this will make more difference than the gear…
Make sure everyone has a blast!
A couple of Grandson putzing ‘round the backyard! Shot with a flash on a bracket wearing a Fong Lightshpere. Flash on ETTL. 70-200mm. Shot at his level. I told Mom not to dress him in a patterned shirt! So much for anyone listening to anything!