When we got to the airport for our first flight, the ticket agent told us that there was an electronic note on the reservation.  She called the gate and was informed that there were no peanuts being loaded and that the plane had been cleaned with his allergy in mind.  The flight attendant informed the passengers via announcement at the beginning of the flight.  Wow.  

After a layover in Atlanta, on our second plane the flight attendants came by to discuss with us that there were peanuts on board and asked us how much "buffer zone" we were comfortable with.  A few rows in front and behind were agreed upon.  Our area was wiped down thoroughly in case someone had eaten peanuts there earlier.  We got to our destination without incident.

On our way back I again spoke to the gate agent.  She got on the plane and examined our seats.  She came back to tell me that she had found peanuts down in the seats and needed to move us.  She then went back onboard to clean our area thoroughly.  On our final flight home there were no peanuts served, more special cleaning, and the captain announced to passengers that we needed to be as peanut-free as possible.

Was every flight consistent?  No.  Was it 100% peanut-free?  No.  Did we feel that Delta did everything within their power to keep our child safe?  Absolutely.  I don't know if we'll choose a peanut-free airline next time, we probably will.  But I have to say that Delta really impressed us.  It was clear through our communication with each and every member of their team that this was a priority for them.  

We now know that there are peanut-free airlines:  American, United, Northwest, Jet Blue, Spirit, and ATA.