Let's say we want to put together a little first-aid kit for home and travel for our newborn. What items would you suggest to be in there?
Hopefully you'll never need to use such a thing, but it is a good idea. I would think all the regular things in terms of bandages and stuff would be a good plan, plus something to reduce pain and fever (e.g. Tylenol in the US or Paracetamol in Germany) AND when the baby begins eating solid foods, Benadryl (liquid) in case of a serious allergic reaction. A trickle poured down the throat before it swells up and restricts breathing can make the difference between healthy easy recovery or not. It can be awhile waiting for paramedics to arrive. The liquid Benadryl was recommended to a group of teachers taking first-aid/CPR training updates I was a part of when I trained for preschool. The suggestion did not get brought up again in later years. This was because laws changed about the ability of teachers to administer such an emergency stop-gap without an explicit OK from the parents, but I'd bet it would still apply in your situation.
Specific to the baby, I would include formula - 3 days worth, (even if he is exclusively breast-feeding, this will do in a pinch if mom and refrigerated milk are both unavailable for some reason), extra diapers and wipes.
Here is a list of resources from the Red Cross on emergency preparedness for families AND their list of minimum supplies one should have available in case of an emergency. This is probably over-kill for what you would carry with you in the diaper bag, but the list includes a link to "anatomy" of a first aid kit for a family of four which I will also include here that should also prove helpful.
You might also double check with your pediatrician once you have YOUR list pared down to see that your pediatrician doesn't think anything important got forgotten during your next check-up.