Hi, bit late as you will have been to the shops now! As others have said it depends on the location and the circumstances, and I have done it at garages, cash machines etc.
However, as I understand it, the law is that you mustn't leave children alone where they are at risk, ie some harm could come to them - even if no harm actually results, you are still committing an offence. There is no minimum age set in England for leaving kids alone.
My main worry would be that a police officer (or "concerned passer-by") would see and cause trouble - a greater risk than something happening to the kids, I reckon!
Car seats are pretty safe these days, but be a little bit wary of the booster type seats used with a normal seat belt instead of a harness - a family friend lost her little girl when she strangled herself trying to get out of one of these, and the belt must have pulled tight. Obviously it was caused more by a faulty belt than the seat, but it's worth bearing in mind.
We all take chances (me included) but I'm going to make an effort not to take so many!