If you want your child to watch your iPad or tablet whilst you are driving, and want to attach the iPad to the seat, we recommend the Universal Tablet Headrest Mount from Halfords.
Its simple and easy to fit and costs £14.99. We fitted ours literally in 3 minutes, albeit the instructions were poor.
It works really well, and when not in the car can act as a support for the iPad too. The Clips fasten to the headrest chrome supports and can clip in and out literally in seconds. The grab mechanism holds the iPad securely and the rotate screw allows you to tighten the alignment precisely.
Keeping your child entertained for that long journey has got to be worth this investment! You can pickup the Universal tablet mount for £14.99 in store or online at Halfords.