The character you are looking for could well be SUNSPOT.
Based on the information you provided, the mutant kid:
- Is Pyrokinetic
- Manifested his powers when he was assaulted
- Boy (between the age 6 - 10 years)
- Comic story released/read in the 90's
Shortly before his fourteenth birthday (as best can be determined; there are conflicting references) Roberto was playing a soccer game with his team (the Thunderbolts) against their arch-rivals, the Dynamos. Racist members of the Dynamos assaulted Roberto during the game, and in response Roberto took matters into his own hands, initiating a fight on the field. While taking a brutal beating, his mutant powers manifested, suddenly transforming him into a creature of solid black solar energy. Surprised and terrified, people abandoned the stadium, and only his girlfriend, Juliana Sandoval, refused to abandon him.
There are some differences though. He is not a kid but a teenager. Instead of the mob, he was attacked by the opposition football team-members.
Although some points are not exactly the same, they do share a lot of similarities. So, this mutant kid you're looking for could be Sunspot.
And you don't have to rely only on Wiki. The Marvel Graphic novel Issue # 4: The New Mutants presents his origin. Here are 4 pages that suitably reiterate the scenario your describe: