D.W.
It clearly states on the Benadryl website that the Children's Benadryl is not for use in children under 2 and while you can find dosing info for a child your little one's age I wouldn't dose it out except under the direction of a doctor.
I have a terrible time with motion sickness. Things that help me are lemon candy, ginger, having air blowing in my face in the car, looking outside and not focusing on things in the car like my phone, reading or a dvd player. If you have a dvd player pointed his/her way I would take it out.