Korean air literally has tons of seats unallocated, like 1/4 of total seats. So get to the airport early or try to call early in the morning on your flight date. Or, call KE now and ask them to leave a memo about your situation so that ground staff will know that before checking-in for that flight begins. This thing usually works out very well.

In the worst case, flight attendants will ask passengers to move around. (Yes, most of the time, you don't have to ask to the other passengers by yourself.) At the end of the day, people don't want to sit next to an unattended 2-year old baby. They will rather go middle seat.