Any type of seat cover poses a risk to your expensive leather. My children are older now and live far away, but I always had great success transporting them in roof mounted carriers, such as the Sears "X-Cargo". They are constructed of light weight plastic, to prevent any "messes" from dripping onto your roof, and they can be locked to prevent any sicko from stealing your precious offspring. The larger model will hold up to three small kids, according to the ad on the box! Of course, for longer trips, you should provide fresh air ventilation.
I used to use the trunk until the car makers started installing those "inside trunk release" cords. I don't know what the manufacturers were thinking when they came up with THAT one!! Go figure, they put child proof features on the rear doors so junior can't open them, but they give 'em carte blanche when it comes to opening the trunk from the inside!!
-Steve