Take care of Seating ASAP
You will not be allowed two lap children in the same side of the same row (not enough oxygen masks). I don't recall whether the 319 has 4 masks in all rows or just one side. If it's all rows, both sides, you can sit across the aisle. If it's just one side, you can sit in successive rows.
But, all of that is dependent on seat availability. That is best handled sooner rather than later.
Having traveled with infant twins as early as at 4 months, I can tell you have having at least one extra seat is a real plus, particularly on a longish flight such as DEN-MCO.