I have been traveling from Brooklyn to the mountains (and that's about 2:30 drive) the past 4 years with all different ages and all different times but nothing helped, just say Tefilas Haderach very erlich, this summer I went with a 4yr old 18months and a 6 months when one was good the other one wasn't, but with one kid a bit easier, but 8hrs, Good Luck!