In an email reply on behalf of Ken Livingstone from Kevin Austin, Head of Transport, the Mayor has stated that he believes all school children should use public buses to get to school, simply because they are free.
In response to an inquiry by a Canning Town resident about why the Mayor does not implement US-style yellow school buses in London (a policy put forward by Policy Exchange and the Conservative Party), the Mayor's office responded that such a scheme was not "cost effective" and it was more "effective to allow children to travel free on the existing bus network."
However, parents of children cannot travel free, and two bus journeys a day equates to around £40 per month. There are many families that simply cannot afford that sort of extra expenditure. As the Mayor offers no solutions for them we presume he thinks they should put their 6 year olds on to public buses alone.
It seems for the Mayor that cost effectiveness is more important than the safety of school children.