The problem here is DL's IT. Since there was an equipment change, the FC seats were reshuffled. The IT doing the reshuffling should have recognized the child and kept the child and mother together. The IT really should do this for any party traveling on the same PNR if possible.
I try my best to accommodate anyone asking me for a seat change, but on a long flight I am not going to a seat I find less comfortable (like moving from the exit rows I always choose on a TPAC).