Agree with Chris. As an ex Londoner, if you are staying in London and 'doing the sights', it will be much more practical to use public transport or cabs. However, if you have a push chair for your toddler, these can be a right pain on the bus, and not much better on the escalators for the tube. You can get short term season tickets for public transport which will save you some money, but I would strongly advise against driving yourself due to all the charges involved. If you get out of the city, there are lots of reputable hire firms.