There would be a difference between 1 December, in which case you should ring, and say 21 December. In the latter case there will typically be a number of people moving or cancelling seats, so another opportunity should arise, or the curtain will get shifted. As it is, you can have one adult in 10A and the other in 10F. If the child is very young then it should be possible to find a combination where one person can move to an equivalent seat without causing any drama.