It's better if you put together an itinerary that includes the things you would like to see and then other posters will help you out it into order and tell you how to get from one place to the next.

"Oyster with train / bus makes more sense or Hop on hop off ?"

"Hop/hop off" are (expensive) tourist buses, for daily travel Oyster or Travelcard, depending on your needs.

Too bad you won't have more time - London is catnip for the hobbits.

Given your time constraints, you need to figure out what you want to see and just drag the progeny along with you. St Paul's, the Monument and the Tower are all fairly close to Blackfriars. Check out the daysoutguide.co.uk website and get the 2for1 vouchers and use a single-day travelcard (buy at Blackfriars or other national rail station) so you can save about 36 quid ($58) on admission for two adults to St Paul's (15 GBP per adult) and the Tower (20.90 GBP per adult).

What time do you arrive - and where? What time do you leave and from where? But 1.5 days w/ a 5 year old . . . You really don't have time to see/do very much.

Pick a couple of major sites you want to visit and if the weather is nice consider either a boat ride on the Thames or maybe visit the Princess Diana Playground in Kensington Gardens.

But we really do need to know more details about how much time you actually have.

Thank you so much BigRuss, Janisj, Hooameye & Cathies ...

I am arriving on a Sunday afternoon and leaving for Edinburgh on Tue noon. So its really one evening and a full day with us. Wish we had more time but it will be essentials for us this time around. The standard tourist attractions, no museums, no shows sadly... Daysoutguide is a great idea, will check. Appreciate your help ! Cheers

PS: Apologize for the late revert ...

@BigRuss ... the 2for1 options are quite limited, do you think it makes more sense to go for a London Pass?

>>the 2for1 options are quite limited, do you think it makes more sense to go for a London Pass?<<

Not quite sure what you mean by limited. Most of the major for-pay attractions are covered by the 2for1's. Some are excluded - but MANY are included.

On the other hand, the London Pass is extremely expensive and a total ripoff for the VAST majority of visitors.

I'd agree with the above - for a 5-year old, probably the Tower (go first thing in the morning), a river trip (you can get a boat from right beside the Tower; some services also stop at Blackfriars pier). Perhaps a stroll along the South Bank around County Hall/Waterloo Bridge (plenty to catch a child's eye there, and some running around space), or around Trafalgar Square and into St James's Park and up to the Palace. I'd have though a must would be a ride on the top deck of a bus (the 15 takes you past a lot of sights and near Blackfriars).

London transport - tube and buses is free for a 5 year old - so no need to pay for other 'private' services. When entering or exiting the tube have the attendant open the gate.