Up to 50% off all apollo bikes - trawl through and see what you like. Also 50% off any cycle helmet when bought as a package.
As they do anyways this includes: Free bike build, 6 week inspection and accessories fitted for free.
I know that Apollo aren't exactly a well renowned brand, BUT - for the kids for christmas for knocking about the street etc they are more than adequate. If granny or grandad has offered to buy a cheap bike it would be difficult to go wrong.