We were lucky enough to visit London on August Bank Holiday weekend when Notting Hill Carnival was on. I would not recommend taking anyone under 6 as pushing a pram in a large crowd with confined spaces is a bit unnerving. It is also extremely loud when the floats go past and even Hannah, my 6 year old was a bit frightened. The tube at Notting Hill is normally closed off so the best mode of transport is the bus.
However the general carnival experience with the glittering costumes, throbbing base lines and party atmosphere was one not to be missed. You cannot fail to jig around to the music and when the sun is out, it is one happy place. We were also glad to come away just before 4pm before it became too hectic for the children and the serious party animals moved in to celebrate.