This is hypothetical really because the parents are both quite flattered & it's a lovely picture
I know it's not illegal to take a picture if a child in a public place but surely if you take a pretty close up photo of two boys playing football to illustrate an article about grass roots it's good practice to ask first.
I personally think they're trying to avoid model fees (the boys are very photogenic). It's a national magazine given out for free in gyms & sports centres I think.