Visiting London ..Pls help?

I’m planning a 10 days visit to London with my family (We are two adults and two kids aged 9 and 4 yrs).We will book online the museum and attractions ticket before our arrival. We have not opted for London pass as we find it not worth if we don’t cover many attractions in a day. Which card should I need to buy if we need to travel by bus or tube. Is bus travel faster within city? What about tube? Can we buy the card and start using right from Heathrow airport. Is it easy to travel and see the attractions if we stay in Fitzroy square near warren tube. What are the must see attractions in London and which all attractions can be covered in a day…I don’t want it to be too tight as we have two kids. A hop on hop off bus is a good option to see, which is the best tour bus company.