I have couple of kids in that age range as well.
I packed some clothing, and a flashlight.
However, I mostly put stuff in their bags that would keep them occupied, and quiet. I carry the bulk of the necessities.
Hello from Missouri. Is bugging out an absolute must for your son and his family? With kids that small, you're talking a major nightmare if they have to go it on foot. The adults will need to be able to carry their own pack, a small child and the small child's needs at the same time for any given distance, terrain, and speed. I would really consider bugging in if it is any option at all. Just my .02.
Your BOBs should have a separate individual "Personal survival kit" with things like a fire starter, a knife, bandana, must-have-medications, and other small bits that are focused on your AREA, SEASON, and anticipated ACTIVITY. These should fit in a fanny pack or similar mode of carry that WONT get left behind EVER. This is slightly different than a BOB, but should be part of it, IMO.
In the actual BOB, have at least one double for EVERYTHING in your personal survival kit. This bag should have other, more long-term and general use types of items. Everything from a collapsible cup(That can be used for boiling water etc) to even other containers, a hatchet, a gun & ammo, para cord, a tent, etc.
The age and physical fitness of those carrying the bags should be considered also. The weight or contents(Like weapons or knives) might be reviewed depending on who is carrying what bag. Get a couple or few good survival books, and keep them in a water prrof container(Like a plastic bag) in the BOB that can be referred to along the way. Even if everyone in the party has read multiple books on it, it's good to be able to refer back to things that were forgotten.