I've just got back from London (was away with work). I've only ever travelled through London, or been to one specific venue/hotel for a night with work...I've never 'done' the touristy London thing so am a novice and know there are a lot of Londoners on here 
I definitely now want to 'do' London with dh and the dc, aged 7 and nearly 5. The really typical touristy things...NHM, boat ride on the Thames, Tower of London, Hyde Park...that sort of thing.
Ideally I'd like to spend two nights/3 days. It's a 3 hour train ride from us...so I'm thinking the dc will have an illness from school one Friday (
), we'll go up Friday morning and come back Sunday night.
What's worth doing and what's a waste of time? What's free/well worth the money and what's ridiculously expensive and avoidable?
Also, where could we stay as cheap as possible for two nights, reasonably accessible to tubes so we can get around...without spending £300 a night?
I know we won't do it on a shoestring but nor do I want to spend the equivalent of a week abroad on a two night stay 
Oh and for the sticklers...AIBU to ask for advice on here? 