[Closed] Travelling round That London

Boris bikes.

That was my 1st thought but according to Mrs Wombat it's "too far" from Kings Cross to the Science museum

You need individual Oyster cards or contact less debit card.

Though personally I'd take the black bus, best way to see London.

Travel cards would be cheaper I imagine; if you have to get 4 oyster cards (each traveller needs their own oyster card), there's a £3 deposit on each one. If you have a contactless payment card, you could use that for one of the adults (unless other adult has one as well). You swipe your contactless card at the gates, same as an Oyster. That would be cheaper than Oyster (no deposit) and will top out at the same price as a travel card (or less if you don't travel as much as intended).

If you're just going direct from kc to the museum it's one tube ride each way (Piccadilly line) so you'll just need a tube ticket.
Contactless bank cards work the same as oyster (at the same rate) so that would be your cheapest option without paying a deposit for an oyster card.

^^ this.

Oyster no longer needed though, contact less cards can be used now. Busses no longer accept cash though. Travel cards have to be bought from Tube stations and are quite restrictive. If you know where you are travelling to them they can be an option but honestly contact less cards have superseded Oyster and you don't get the discount you used to either for using Oyster.

Boris Bikes are a good way of seeing the City but if it's raining you'll need the Tube or Bus. Bus routes are difficult to follow and subject to diversions etc. so I'd suggest one of the big Tour Busses to take you from tourist spot to tourist spot.
Or Taxi..

If you're only going for the one day and unlikely to visit as group often than I'd probably go for the one day travel card option.

Having said that I'd also suggest walking as much as possible, If you're travelling less than 3-4 tube stops then taking the tube can often be longer (and far less pleasant/interesting) than walking. Mapping app on a smartphone can help no end with navigating your way around London.

If you're staying central, rather than travelling in from the 'burbs and think you're going to be focussing on one or two areas then you might be able to abandon tube travel altogether and get black cabs to cover the main bits and walk the rest. Between four of you £36 would buy you two or three short-medium length taxi rides.

Contactless cards are fine, but same as Oyster, you'll need a different one for each person. Repeated swipes with the same card at the same gate will confuse the system and you'll probably wind up getting charged the max fare.

Thanks for the quick replies.

Based on this it looks as though we'll be going for the adults using contactless cards and getting travelcards for the children.

We will be walking a fair bit but both junior Wombats regard tube travel as part of the London experience

I have an oyster but always use a contactless bank card now. Fwiw, it will always charge you the lowest fare automatically and debits a couple of days after travel.

How old are the kids, if under 11 they travel free with adults using oyster/contactless.

Oyster will be cheaper than travelcards for the kids too. You can always return them at the end of the say to get the £3/4 deposit back.