Further to Abdulmajid's answer:
Unless the children were unclothed, it's unlikely that any prosecution could be brought under the Sexual Offences Act (or any other legislation). A recent test case, for example, on a charge of 'voyeurism' ruled that it could only relate to exposed parts of the body which could normally be expected to produce sexual excitement. (A guy who filmed bare-chested men in a changing room was acquitted because male chests weren't considered to be sexually titivating). As far as the Sexual Offences Act relates specifically to children, it refers only to 'indecent' images. (The test case of R v Oliver ruled that images of 'partial nudity' could be considered to be 'indecent', but this probably only relates to, say, pictures of a topless female teenager. Pictures of children playing in their swimming costumes would be unlikely to qualify).
Since the question refers to a criminal offence, I'm surprised to see Abdulmajid's reference to commercial enterprise. Using someone's image, for commercial purposes, might leave the photographer (or publisher) open to civil action but it's not a criminal offence.
Harassment is hard to prove. It requires showing a sustained course of actions likely to cause fear or distress. (Simply annoying someone by repeatedly taking their photograph - even when asked not to do so - is not, per se, an offence).
Otherwise, as Abdulmajid states, anyone is generally free to photograph (with both still and moving images) who or what they like, as long as the photographer is in a public place (or on his own property or elsewhere the with permission of the property's owner). The position of the subjects of the photographs is irrelevant. It's lawful to photograph children - or anyone else - in the street (or even through the windows of their houses) as long as the photographer is in a public place.
Chris