They will fix the bassinet for you, but normally it is not long enough for a baby of that age.

My advise: Take to aisle seat for you and your +1 in the centre row and try to get the middle seats blocked. SQ may do this on request if flight is not fully booked. The baby sleeps better on the seats and you wont have the "please-take-the-baby-out-of-the-bassinet-as-we-experience-turblence" game which is popular among SQ pilots on longhauls.