Kids mostly make a mess with food. The simple solution is not to give them food or drink in the car. If you 'train' them like that from the start, it'll be a good few years before they realise what's going on. The only think that I did was have a tea towel under the car seat.
We had sunshades in both cars. In fact, I kept them in until the eldest was about 7 or 8 as I liked the tinted look, until they complained about having 'baby shades'.
