The Home Office has warned parents they must take responsibility for ensuring the photographs are kept up-to-date.
Mike O'Brien, the Immigration Minister, said the move was primarily aimed at reducing the number of child abductions and improving security at ports.
The latest Home Office figures reveal 374 children were illegally removed from the custody of a parent or guardian and taken abroad in 1996. Some parents never see their children again or become involved in expensive legal actions that can take years to resolve.
Last month, Jamel Bain, whose Algerian father abducted him eight years ago, was finally reunited with his mother from Scotland. Nearly 16 years old, he had forgotten how to speak English. Denise Carter, director of Reunite, the National Council for Abducted Children, supports the new passport law.
"Child abduction is a growing problem, partly because of a greater number of cross-cultural marriages, and the fact that international travel is so much easier these days," she said. "A lot more people have children without getting married, which further complicates matters."
The new law will immediately affect more than 500,000 under-16s applying for passports for the first time. Those already on a parent's passport will be exempted until it expires or the child reaches 16, when their own passport becomes compulsory.
A Passport Agency spokeswoman said parents must take care that the inevitable changes in appearance of a growing child do not make the passport photo redundant soon after it is taken.
"Obviously a six-month-old baby will look very different a few years on, and it is incumbent on the parents to make sure they get a new passport if necessary," she said.
Mark Harris, a travel agent in north-west London, said families could be refused entry to their holiday destinations. "People usually laugh at their passport photos because they look so different from reality, but young children change so quickly it could be the recipe for some travel disasters," he said.
The design of the new child's passport is the same as the adult version, but it is valid for only five years rather than 10. It will cost pounds 11 by post or pounds 21 for personal applications.
Jackie Gibson, of the Association of British Travel Agents, said its 7,300 members had been warning travellers about the changes. Abta had been concerned about the extra costs posed to family holidays: "We were worried about the extra expense, but the reality is that most European countries now issue children's passports and we don't foresee too many problems."Reuse content