You can try
promethazine,
diphenhydramine, hyoscine (with caution as studies have only been done in adult males - though it is less likely to cause sleepiness),
cinnarizine.
Simple ways to prevent travel sickness are -
Focus child's attention elsewhere, e.g. out of the window, on the horizon where practical
Do not encourage reading or focusing on games while travelling
Avoid unnecessary head movements by using pillows or a headrest
If travelling by car, seat child near the front of the vehicle, that is, middle rather than third row in a larger vehicle
If flying, sit over the aeroplane wing – the ride tends to be less bumpy
Have the child recline as much as possible
Feed the child a light snack before travelling – avoid heavy, greasy meals
Ensure ventilation either from open window or air conditioning – avoid overheating
Try to keep calm –
motion sickness is more likely to happen if a child is worried about having an episode
Hope this helps