Are the Greasers in The Outsiders kids or criminals?

It really depends on how one looks at the events in their lives. The Greasers share a common background, and the element of hardship is what brings them together. They are young, and life is tough on them.
Darry, Sodapop, and Ponyboy are orphaned at an early age and left to fend for themselves. Darry and Sodapop are forced to drop out of school.
Johhny is abused by his parents and finds solace in the gang. He stabs Bob to defend Ponyboy. He also helps rescue the children in the burning church.
Dally is a delinquent with a long rap sheet. He keeps a gun and is not afraid of causing grievous bodily harm to others as is seen when he takes the switchblade with him to the fight.
The Greasers are just a bunch of kids from difficult backgrounds, and their participation in criminal activities is circumstantial. It is my opinion that in different circumstances the boys would have turned out differently. They lack guidance and a sense of direction.

The answer to your question, of course, will depend upon your interpretation of the Greasers' words and actions. Many consider a group such as the Greasers a gang of juvenile delinquents.
However, the Greasers are an important social barometer of their time; the majority of the members come from impoverished, dysfunctional families. Ponyboy Curtis (the narrator of the story) is a Greaser, and his only biological family members are his two brothers, Sodapop and Darry. While the brothers love each other, they often experience conflict in their domestic lives. The boys are orphaned, and Darry is the presumed father figure and family provider.
However, Darry is only 20, while Ponyboy is 14. The latter often resents what he considers his older brother's efforts to meddle in his life. However, Darry (at his wit's end himself) is merely trying to keep his family together. The other members of the Greasers gang (Johnny, Dallas, Steve, and Two-Bit) also come from dysfunctional family backgrounds. For example, Johnny suffers emotional and/or physical abuse from both his parents.
The Greasers are at odds with the Socs in the novel. The Socs are another gang, one made up of upper-middle class youths. Invariably, the Greasers fight the Socs for two reasons: they enjoy fighting, and they see it as their duty to defend themselves against attacks from the Socs. 
The one Greaser most associated with being a criminal is Dally. In the novel, Dally openly revels in his gang background from his New York City days, and he is the one who robs a convenience store after Johnny's death. No one quite knows why Dallas chooses to commit a crime, but there is every indication that he was deeply affected by Johnny's death.
So, on the surface, Dally appears to be a criminal. However, he is also an adolescent. In his short life, he has never learned constructive ways to process his grief and anger, and he has never envisioned a different life for himself (one beyond the criminal world). So, is Dally a criminal or just a kid? It really does depend upon your perspective and how you think we should treat kids who commit criminal acts.