I have a professional link to childcare services in that locality and the information issued by the police is that it isn't true; there was an alleged incident about a young girl being approached whilst sitting in her parents car but not what was described here and in many other south London community and baby websites. Clapham Common was not in fact flooded by police in respect of such an issue last week. If they attended it may have been for some of the other notorious things that the Common is famous for!
There is a lot of stuff written about Clapham Common and I believe lots of 'abducted by aliens' stories and even websites for people who go there hoping to abducted by extra terrestrials so I would take things about this area with a pinch of salt. However its always very wise to be careful with one's children as bad things do and can happen hence vigilence is always a good idea.
And a pair of toddler reins.