IMO
Considering a 400mi. trip with your daughter and not your husband??
Will you be packing everything, can your daughter pack anything?

No way that you will make the trip without re-supply. I would suggest two
re-supply points at a minimum. Can you locate cache points now and stash
some water, food, fuel, socks etc. I am thinking this trip will take at least 3 weeks
but more like 4.

Don't pack more than 2 MRE's total. For a bug out IMO they are ideal, almost a survival
kit in there own right it sucks that they weigh so much. You need to switch to FD food.
FD is not as bad or expensive as you think, just be selective on purchases. I Like Chili Mac, it's not to spicy, a 2 person meal for $6.00. Most of the meals for my taste are a bit saucy so I also add extra instant rice to "help" and bulk them up. Instant oatmeal is another standby, so are oatmeal cookies, trail mix, nuts, etc. etc.
Trail menu's can take an entire thread.
Check Wal-Mart and REI Outlet for food deals.
Still for 400mi. no way will you be able to pack enough.

On the gear:
I would drop the folding shovel and use a garden or folding trowel, heck even a GI mess kit spoon makes a good shovel.
Forget the flashlight and switch to keyring lights and keep the headlamp or just use only the headlamps. Still nice to have one keychain light on a easy to find zipper tho.
Those firestarter matches are handy but do you need that many? Only use them in a real emergency, not just cuz you want a fire bcuz you need a fire!
Collins Gem is a nice survival guide about 1/4 the size of what you have . Is it as good,
only you can decide.
Forget the mess kit and switch to a small coffee can HD rubbermade containers 1 or 2 cup size storage bowls with snap on lids for use as cups, bowls or storage. Having a small
plastic measuring cup is handy and only weigh's an extra oz.
For a knife perhaps a SAK Rucksak or slightly smaller Camper, both have saws.
You need to add a sleeping pad (s). to your list.
A Tarp is more versatile than a tent, and I happen to like sq. cut sleeping bags bcuz they can be used as a bag or blanket.
A stove will be tough, you need fuel and lots of it for 400mi. If using a lp gas stove
I would guess a min. of 4 1lb cans, ie 1 per week. IF using a liquid fuel stove plan on a min. of 3/4 gal 1 gal more likely. Either type of fuel will be more than you want to carry. I personally can get by with a full Svea stove + 1.5 pts. for a week.
Perhaps a Emberlit stove could work for you, still it weights 11oz

And lastly don't forget that pack weights 6 pounds empty, perhaps consider a different
pack down the road. A HD pack can be had that is under 4pounds, save that 2 pounds for supplies.

And the list can go on and on.

Almost forgot, don't forget to pack a few HD trash bags, they have to many uses to list.
Shelter, rain cover, wind protection, storage are just a few of them. At least 1ea and one spare.

If you're in a situation where you have to walk 400 miles to find safety, I don't think pepper spray is going to do much for you in the way of defense. That stuff barely works (if at all) when you can quickly spray an attacker and then run for help and call the police. That's not going to be the case in the scenario described. I would take an extra loaded mag for the .40 over some pepper spray. The .40 should be in a holster on your hip too, not buried in the backpack.

Actually, I think the OP has done a lot of good thinking and researching on the BOB. Planning to walk 400 miles is maybe a bit over the top in terms of realistic expectations though. Especially with a 6 year old.