In the UK, no offence is committed, however many public locations cite the Data Protection Act 1998 as a reason to stop people taking pictures.
DPA does not mention this topic at all, and is a red herring (however informing the location of this is unlikely to help, I have discovered)
In fact, in the UK, the only laws that appear to exist cover either specific locations and properties (eg military installations) or using photography to take pictures of individuals in areas where they have an expectation of privacy.
The Photographer's Rights Guide published by digitalcameraworld in 2012 is still current as far as I can see. It has this specific guidance:
Photographers Rights: Taking Pictures of People in Public
Are you breaking any law when you’re taking pictures of people in public? Probably not, but the position under UK law is uncertain.
There are currently no general privacy laws under UK law, but the UK courts must take into account the European Convention on Human Rights, which gives everyone the right to respect for their private and family life. As this is an area of law that has been developing rapidly over the last few years, it is hard to be certain what will constitute an infringement.
The key issue is whether the place the image is taken is one where a person would have a reasonable expectation of privacy. For example, it has been suggested that the right of privacy of a child could be infringed by publishing a photo of them with their parents in a public street.
It is therefore advisable to be careful when taking photos intended for publication, even where the subject matter is in a public place. Failure to obtain a model release for the use of an image will certainly make it harder to sell the picture to stock libraries.
The same laws apply to adult and child subjects, but a child does not have the legal capacity to consent and a parent or guardian must therefore do so on their behalf. Be aware that schools, leisure centres and places where children and adults gather usually have their own photography restrictions.
Although decent photos of children (see our tips for better pictures of babies, children and teenagers) taken in a public place may be fine for non-commercial use, seek permission from the child’s parents or guardians and don’t shoot covertly with a long lens. For commercial images, you’ll need to get a model release signed by the parents.
Also read the section on the powers of police and security guards.