Contrary to growing popular belief (driven by paedophilia hysteria), there is no law (yet) which prevents anyone from taking or using a photograph of any child provided it is not indecent, or manipulated in a way to make it indecent. The Protection of Children Act 1978 does not define indecency and leaves it to the jury to interpret.
Deliberately taking photographs of children in a public place will draw attention to you and possibly arouse the interest of the police; in Scotland you could be committing a breach of the peace. Many organisations will also have policies on photographing children at organised events; even if the children are your own you should find out what rules are in place before getting your camera out. It is always a courtesy to ask a parent’s permission before photographing their child, but not legally mandatory.
Potentially, photography could be considered harassment if, for example, you were to take photographs of someone against their will, and as with other forms of harassment only two incidents are necessary to constitute a course of action. The European Convention on Human Rights also protects an individual’s right to respect for his private life, and breach of this could be an offence. If for example you were to take a photograph of your child in his home using a telephoto lens from a location outside his home, that would be an invasion of his privacy; so too might be taking a photograph of him in the street, depending on circumstances. As a child does not have the legal capacity to give consent, the consent of a parent or guardian must be obtained in writing.
It is entirely legal to post photos of your child on a website. The only offence would be if you identified your child as being the subject of court proceedings – that would breach Section 97 of the Children Act, although no known prosecutions have followed. The solution is probably to put photographs of your child onto someone else’s website and deny that you have any knowledge of them and that you have no control of who puts them on the site. That way you frustrate the judge, you annoy the other parent and there is nothing they can do. But if you have done nothing wrong why react at all?