Competition for this multi-million-pound market is growing, with Thorpe Park in Surrey deliberately undercutting its neighbourhood rivals - the new Legoland in Windsor and the longer-established Chessington World of Adventures.
They all charge prices that - many families may feel - could do with cutting.
Thorpe Park has kept its admission fees at last year's level, which means a day out for a family of four costs pounds 49 - pounds 5 cheaper than Legoland's pounds 54, and pounds 10 cheaper than Chessington's pounds 59. If tickets are booked in advance, a day at Thorpe Park comes down to pounds 40.
Theme parks are big business in Britain, with 12.6 million people visiting one each year, netting the parks pounds 170m. The average admission fee was pounds 16.50 but 40 to 45 per cent of parks' income comes from the "extras" - food and souvenirs.
But they face increasing competition from other entertainment outlets, such as the new SegaWorld video-game centre opening in London's Piccadilly, and an inside theme park at the Metro Centre, Gateshead, Tyne and Wear.
Older theme parks, such as Blackpool Pleasure Beach and Alton Towers in Staffordshire, have replied by investing in larger, more intricate white-knuckle rides. The Pepsi Max Big One rollercoaster at Blackpool cost pounds 12m and has a drop of 235 feet at an angle of 65 degrees.
Even London Zoo is getting in on the act, employing Lego's biggest rival, K'Nex. Children will be able to see giant animal models, including tigers and orang-utans made out of K'Nex, described as a cross between Lego and Meccano. K'Nex is eager to point out that its attraction will cost pounds 7.50 for adults and pounds 5 for children - much cheaper than the theme parks, and the price includes a day at London Zoo.
Chris Edge, general manager at Thorpe Park, said: "The UK leisure market is becoming increasingly competitive and we have decided to take a bullish attitude and fight them head-on by cutting our prices. It can only be good for customers."
But both Chessington, which charges pounds 16.50 for adults and pounds 13 for children, and Legoland, at pounds 15 for adults and pounds 12 for children, said they would not alter their prices to compete with Thorpe Park, preferring instead to invest in new rides.
Legoland is hoping to attract 1.4 million visitors a year and is incorporating miniature versions of St Paul's Cathedral and a Tube station, as well as areas where children can construct their own models.
Joanna Oswim, head of marketing at Legoland, said that prices had been fixed after extensive research. "Our target market is ABC1 families. We have put in an investment of pounds 85m so it will be quality product. It is not our policy to discount at all. We have very few sales promotions. We would rather add value to a day out."
Chessington, which attracts 1.7 million visitors a year, has launched a new ride known as Rameses' Revenge, a 60ft ride that goes through 360 degrees. Admission charges are pounds 16.50 for adults and pounds 13 for children.
Virginia Sudakiewicz, marketing manager for Chessington, said: "The competition is based on what we are planning to do, not on other people. We set our prices at good value for money."