christmas lights n stuff in london

[FoJo]

we are in Newcastle but coming to London for a few days the weekend before xmas, me dh & dc's 13 & 16. We've done London quite a few times but never at xmas. We are having a day at harry potter but looking for the best lights and so forth to see. Maybe something in the evening? I see there seems to be stuff in Hyde Park & Kew Gardens, any thoughts on which is best? Thanks