We took Rosie to spain @5mths, america @10mths, and Max just come back from america @9mths, and he went to spain @4mths. one thing, i would get her own seat as a 18mth old on your lap for 4 hours is gonna kill ya!! ( and not to mention you will have room to spread out, and her luggage allowance! we never took any car seats on board! )

HTH's.