You can ring BA and see if they will sort out the seating given that you are travelling with a very small child, or at least ask them place a note in your booking to let the flight editor at the airport know to seat you together before online check-in opens. Also, please be aware that seat 20E is a infant cot seat and is usually blocked out of the system for infants. If there is no infant, then it will be released at online checkin.