As seen on the Dragons Den. Tray Kit is a child's carry-on backpack and play-tray which can be strapped to train and plane tray tables and hung on the back of the front seat in a car to provide an extendable play surface with raised sides.
This is the cheapest I have seen these all seem to be in the region of £24 elsewhere.
Found instore at Sainsburys Farnborough and Camberley available in both pink and blue
Info about the item @ http://traykits.co.uk/