We too have grandparents looking after the children while we have a night away for our anniversary....we thought about going abroad, Venice, Paris, etc... then somewhere in England in the country (we have stayed here:
http://www.calcotmanor.co.uk/ and it is lovely. They do have children in the hotel but think going at a none school break time should be fine and they have a luxury spa with heated pool that the children only have access to for 1 hr 15 mins a day. Would highly recommend) In the end though we decided to have a night in London! We never do it, very rarely venturing out of nappy valley so we thought we would cut out travel (grandparents are coming to our house to look after the children) and make the most of our great city! We are going to a lovely restaurant, staying in great hotel and just enjoying ourselves generally...
aa x