cut and pasted from a random site. Hope it helps.
For visitors in London for relatively short periods the main practical reason that makes the Oyster Cards less attractive is what happens when you leave London and need to recover your deposit and any money still left on your PrePay account.
On handing in your Oyster Card, you will need to fill in a form with your home address. You will normally receive any money including your £3 deposit at this time, but at some ticket offices, especially if very busy the sum owing may be sent as a UK sterling cheque to your home address.
If you have children, the choice is further complicated. See the Free Travel for children paragraphs below that may mean you need to obtain a Photocard for your children. Purchasing a 1 or 3 day off peak Travelcard allow your child (11-15 yaers old) to travel for £1 without the need for a Photocard.
Free Travel For Children Under 16
Under-5s
Children under five can travel free at any time on the Tube, DLR, buses and trams as long as they are accompanied by an adult with a valid ticket. Children in this category do not need Photocards.
Children 5-10
Under-11s can travel free at any time on buses and trams without the need for a Photocard.
They can also travel free at any time on the Tube and DLR when they are travelling with an adult who has a valid ticket. Up to 4 children under 11 years old accompanied by a paying adult travel free on the London Underground. Under-11s travelling with an adult will not be issued with tickets and will be allowed to pass through the ticket gates at stations. Again no Photocard is required for children travelling with adults.
Unaccompanied children between 5 and 10 must have a vailid Photocard.
11-15s
Children aged 11-15 can travel free at any time across the entire London bus network, but must have a Photocard.
Children aged 11-15 can travel on the Underground, Overground and DLR for a maximum of £1 a day but must have a Photocard.
For visitors with accompanied children 11-15 years old you can avoid the whole photocard issue by simply purchasing an accompanied child day Travelcard each day for £1. The adult must have a valid Travelcard, this could be a 7 day Travelcard, it doesn't have to be a day Travelcard.
Obtaining a Photocard
Visitors to London can order a Photocard in advance for their children and collect on arrival...