The NIH does a good job of summarizing the basic requirements for obtaining assent from minors. To put a long story short, you'd need to document both: a) written assent from the participant who is a minor; and b) written permission/consent from his or her legal guardian.
Wards (defined as "a child whose welfare is the responsibility of the State or any other agency, institution, or entity") are slightly trickier. The NIH describes research on child wards as appropriate if the research study is "related to their status as wards; or Conducted in schools, camps, hospitals, institutions, or similar settings in which the majority of the children involved as subjects are not wards." In addition to that, during the remainder of the study, "an advocate must be appointed for each child in addition to any other individual acting on behalf of the child as a guardian or in loco parentis. One individual may serve as the advocate for more than one child. The advocate shall: Have the background and experience to act in, and agree to act in, the best interests of the child for the duration of the child’s participation in the research; and Not be associated in any way (except in the role as advocate or member of the IRB) with the research, the investigator(s) or the guardian organization."
Standard protocols of risk vs benefits of research still apply, but depending on the actual age of your participants, the degree of risk will fluctuate significantly, be it physical or psychological.
I have attached a link describing the process, as well as a template of a child assent form from Cornell University.