Given the weight of a 12 week old baby is about 4-6kg I fail to see why you need a stroller at all on the aircraft; just carry YOUR babies. I was recently amused to watch two parents in F on the way to LAX who had decided to have some "quality time" with their toddler; Nanny natch was in Y. Toddler did not want quality time with parents and as soon as seatbelt sign was off made a run for it down to Y. Nanny brought said toddler back to see parents and it was clear that staff were happy for her to visit briefly but child preferred Y with nanny.