Copyright law means that the person who has taken an image owns that image and can choose what to do with it. But are there any rules over what can be taken?
In the UK, taking photos in a public place is not illegal. There is no law stating that you can't take photographs in public. This includes taking photos that include other people's children or taking photos of children directly - obviously photographs that are indecent are illegal. It is wise however to get parents permission before taking photos of their children to avoid any problems of unhappy parents.
A public place is usually a place to which the public are allowed to have access freely and without payment or permission. This includes any public highway or footpath. The inside of a car is also considered as a public place, unless it is parked on private property.
If the person you're photographing is on private land, they could claim a right to privacy, and if you're on private land, then the owner of the land has the right to restrict photography on their property.
Selling or using photos commerciallly
If the photo is going to be used for journalistic or artistic material, it is usually acceptable to sell it. However if you plan on selling the image commercially or use it for a commercial purpose (for example to promote a product) it is normally recommended to get people to sign a model release form, and to get owners of property to sign a location release form.
A model release form is simply a signed form that says the person being photographed is happy for their image to be used in commercial products - image consent. They can include an agreed payment (such as money, or free use of the images themselves etc.). A location release is the same idea just for the physical private location - it is signed by the owner.