It depends on what your children are already interested in, really. There is the Museum of London not far away, and that provides some interest for young children. There is also Sir John Soane's Museum, but that may be a little to old-fashioned for them. That may go for the St Bart's museums as well, but there are some very interesting exhibits if they are at the age where that sort of thing appeals? You are also not far from the river and you could take them out on one of the ferry rides there? Otherwise, a single stop from Holborn underground to Leicester Square takes you to a far more vibrant area with the M&Ms World centre and short walk to Trafalgar Square which may be more in young children's line?

There aren't any park & ride sites that are close enough for anywhere in central London as far as I know.