I keep googling this and can't seem to find a straight answer! Travelling on Monday with my DS who will be 2 in 2 weeks. I've booked him his own seat as he is a bit big now to hold comfortably for 4 hours. Do I need to bring a car seat for him to sit in or will he be allowed to sit on the plane seat? Tui flight from UK to Tunisia, if that makes any difference.