Stupid maybe, but I have been wondering about this recently - what do you do when you go and pay for the petrol you've just put in your car? Get your kids (in my case a 2 yr old toddler and a 2 month old baby) out and take them in with you, or leave them in the car? The first seems a lot of hassle but the latter doesn't feel very safe...
Having just joined a carclub (highly recommended, BTW!!) I am suddenly faced with this.. (normally using public transport I've just never thought about it..).