On a budget: Transport

London is an expensive city but if you plan your visit in advance, you can save a lot of money.
Transport is something that you won't be able to avoid and it is expensive, but a bit of research can halve its cost.
First of all, you can find all the information (maps, routes, fares, etc.) here. I will try to sum up the fare information for you, but if you want in-depth information, the TFL website is perfect for you.

Rule #1: AVOID single tickets
They are a rip off. A single underground ticket (no Oyster / contacless card) in zone 1 costs £4.80! Two single tickets add up to more than the daily Travelcard. Therefore, AVOID.

So, what can you do?

1. Oyster card: It costs £5 (deposit) + as much money as you need. Every time that you use it, the price of the ticket will be deducted until you get to a daily cap (the daily cap is the same as the daily travelcard). At the end of your stay, you can return your Oyster card in an underground station to get your £5 deposit back. It can be used in bus, train, underground, overground, DLR and River bus.
2. Travelcard: A daily transport ticket for as many journeys as you want. You can get it paper or put it in your Oyster card. The price depends on the chosen zone. It can be used in bus, train, underground, overground, DLR and (a discounted fare in) River bus.
3. Visitor Oyster card: It costs £3 (non-refundable) + postage + as much money as you choose. It is cheaper than a travelcard or Oyster card, but it has to be bought in advance. Includes discounts for some restaurants, shops, etc.
4. Bus & Tram pass: It costs £5; it's a daily ticket for as many buses & trams as you want. The hotel is far away from the tram line, but it is still a bargain even if you don't use the tram line.

Now, children:

• Under 11: Free in every transport when accompanied by an adult who has a valid ticket (excluding Group Day Travelcard). On some trains they have to pay a reduced fare.
• 11-15: They can get an Oyster card and get half of an adult rate, but this has to be done when kid and adult are together.
• 16-18: Adult fare unless they have a Zip Oyster photocard (which you have to apply for at least 4 weeks before visiting London). If they have one, they will pay half of an adult rate.

I hope that you find this information useful, since the fare system in London is quite complicated and it takes some time to understand how it works.