I don't know how old the kids are but I think the (free) British Museum in Russell Square is unmissable. Treasures from antiquity around the world....nobody stole them quite as well as Britain.
If they're young kids very nearby there's a reward for suffering the museum. It's called Coram's Fields and it's a brilliant public playground build on some very expensive land! See www.coramsfields.org
And for suffering the playground, for Dad, just across the road there's a great old London pub called The Lamb, which seems to accommodate children.