In my conversations to local parents, I get the impression that the Little Lambeth is a really helpful blog so I thought I'd let you know about some of other blogs which I find useful when it comes to looking for places to take children.
SE19 Kids lets you know what's on in the Crystal Palace area. Great for families in the south of the borough.
For parents in Streatham, check out Tooting Baby's diary of what's on in the area. Also includes a list of baby friendly eateries and local places to go for a day out.
Looking for reviews of things to do in London, then check out London with a toddler, lots of entertaining stories about visits to different places in London. Also includes an ace googlemap so that it's easy to find the places that the author has visited.
And finally, check out Clapham mums which covers the Clapham, to Clapham Junction area. It lists local activities and events and if you sign up, you can get a card offering great local deals.
So there you go, a few more helpful hints. Let me know if you can think of anyone else to add to this list!