A year or so ago, a video went viral: How To Sneak Into Theme Parks. In it, a practical joker apparently managed to smuggle his friend into Disneyland by putting him in an oversized pushchair.
It's by no means unknown for children - and parents - to lie about age in order to bag a half-price ticket. However, you're risking massive embarassment if you're then asked for proof.
So how do child prices work - and is it ever OK for an adult to get a child ticket?
Generally speaking, children under five travel free on trains - although they're only officially allowed to have a seat of their own if no other adult needs it. Between five and fifteen they get half price.
There's no way round it, but you could always buy a Friends and Family ticket, which allows up to four adults and four children to travel together; children get 60% off the usual price and adults 40%.
In London, under-11s travel free on the tube, DLR, overground and TfL rail services as long as they're travelling with an adult or have a 5-10 Zip Oyster photocard. On buses and trams, it's even easier, with no Oyster card needed unless they look over 10.
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In most theme parks, under-threes go free, but over-10s have to pay the adult price. There is a rationale for this - that's the age at which they tend to be tall enough to go safely on all the rides.
At Legoland, kids under 15 get the child price - and one can go free if accompanied by a paying adult. But most flexible of all is Peppa Pig World, where children under a metre tall can get in free; so if yours wear flat shoes and are good at hunching down, you may be in luck.
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This is one place where an adult often can get a child price - though not necessarily. Many restaurants offer particularly cheap child meals as a way of getting adults though the door, and they won't be happy for you to try and avoid it.
Many restaurants offer children smaller portions of the same meals, and this is generally a much better bet; asking for a child-sized portion of pasta, for example, is often perfectly OK.
If a child-sized meal isn't actually listed on a separate menu, you'll generally be able to order it whatever age you are. Says a McDonald's spokesperson: "A Happy Meal is charged as a Happy Meal irrespective of the age group purchasing it; you would be charged the same despite age, and despite who ends up eating it!"
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Odeon offers a family ticket for up to four people, allowing adults to pay the child price. Meanwhile, Vue's Mini Morning screenings on Saturdays, Sundays and school holidays cost from just £2.49 - and adults pay that lower price too.