"I haven't figured out what to do for my youngest & smallest yet."

If you haven't gotten him something yet, at the very least, get him a day pack. There are other threads here on kid's packs, so search and go back a year or so.

Even if you are carrying almost everything else, a day pack should have (weather dependent, of course) some extra clothes (fleece, gloves, hat), snacks, and as already mentioned, minimum ten essentials (whistle, small flashlight, etc.) and a water bottle. Tossing in a cheap space blanket wouldn't hurt (one of the ones in a tiny plastic bag) just in case.