hi daisy - I remember there being a couple of nice arty shops in Long Acre, Covent Garden, so have this link for you which might be useful. St Martin in the Fields Art college is round there. Not sure if they're the right sort of thing, but they might be?
Is this for DaisyBoy's birthday party? I haven't been, but I wonder whether the Covent Garden shops might lean more towards the hand-made Japanese paper end of the market and might possibly be a little detrop for infants, but then I am very mean and do not entrust MBB with my treasured art stuff. [bad mother emoticon]
If you don't want to troll around the shops (although Covent Garden is a good place to troll [disclaimer: not in the MN sense]), then Baker Ross has lots of crafty stuff for small children. These door hangers went down a storm when MBB was three or four. Sticking the googly eyes on needs a bit of adult supervision, but the results can be endearingly Picasso-like in a two-eyes-adjacent-to-an-ear kind of way.