my children want to go to london at half term to gawp at big city bankers, scratch the gold plated pavements and get black snot.

SO in the interests of economy we are looking at holiday inn type thing and Wandsworth is very bargainous at £70 a night for all 4 of us including some sort of depressing breakfast. Is it a pain to get anywhere though?
It is on the overland train route, bit of a walk to underground, would it be cheaper to just get taxis seeing as there are 4 of us or are kids free (they are 5 and 6)?

Is it OK round there? other suggestions are welcome?