depends how rural you want to be! You could stay on the Ayrshire coast without a car easily, try Troon or Prestwick, both have stations with 30 min train to Glasgow plus are nice towns with lovely clean beaches. not rural though. you could also manage Loch Lomond I reckon, but not the top end, down Balloch/Cameron House way. Maybe Helensburgh would be more your thing? These places are more 'normal' than lochs and glens type places though!
You could get the train up to Aviemore, it's lovely up there, plenty local buses to get about once you arrive. Loads for kids to do.
thanks I will look into some of your suggestions, weve been to edinburgh and loved it and would like to see a bit more of scotland , this time more of a seaside holiday, doesnt have to be rural and am not mad on shops, just want a base that is nice with good transport links but where we could happily spend a day pottering around without going too far. we would be travelling by train from london,
Something a bit more unusual/exciting, sleeper to Fort William. Harry potter train to Mallaig, then ferry to one of the small isles, everything within walking distance, fantastic beaches, small shop/tea room on each island, but no crowds, big shops or attractions.