Ford Focus. They can rack up the miles if looked after, cheap to repair and get a small engine cheap to ensure. The best bit is that they have good safety for the age/cost. Mine was hit by a small Jap hatch from behind. The Jap car was a complete wreck...suspension hanging off, loads of panel damage and only fit for the scrap yard. Mine had a broken plastic bumper, a dented inner steel bumper and I drove off and still drives perfect. Secondhand parts to fix £35 and 2hrs!
I would not let my kids drive anything smaller. The other good thing is that they do handle very well so the chances of it crashing in the first place is reduced. I wouldn't knock Fords, my Focus has done 145,000 miles and is totally reliable. I also use a Ford engine in my track car and if looked after they really are very good...and I can still see the clock OK in the LCD display!