Assuming that you are talking about U.S. Law, refer to Title 18 USC § 3509 , part II, chapter 223. This is entirely linked to the rights of children as witnesses in the US Court systems, and prevents their misrepresentation, secures their protection, and enforces their ethical treatment. Let this be the baseline "guideline" that you will use to develop your own personal criteria. The law always comes first when it comes to minors.
Now, since your question asks to define parameters that would help determine the eligibility and validity of the testimony of a child, what you want to do is aim for three very important factors that may seriously affect the testimony:
- the child's understanding of the problem at hand
- the child's ability to verbalize his or her knowledge
- the child's ability to recall information as asked
All of these factors are directly connected to (in the same order)
- the child's cognitive abilities (based on age, family situation, socioeconomic background, and emotional intelligence)
- the child's developmentally0based phonemic and phonetic awareness, as well as vocabulary knowledge
- the child's working memory
A psychologist, particularly a school psychologist, can help determine the cognitive, developmental, and psychological abilities of a student as they relate to the problem at hand. These abilities serve as good predictors of social adaptation. For example, a child that is consistently scared, or feels threatened, is on a never-ending defense mode. This can manifest in cognitive dissonance, which is when kids "don't get" what is really going on in their lives and a "misfire" of information ensues. This cognitive dissonance is what a school psych can assess.
Other factors that you may want to include are:
- whether the child is traumatized or affected as a result of the situation at hand
- whether the parents have given consent
- whether the consent forms (every single one of them) are current and notarized
- what is the academic and social history of the child (at home, with teachers, and with peers; patterns/behavioral trends?)
- what is the relationship between the child and the people involved in the case (conflict of interest?)
This is the basic "checkpoints" that are always to be considered when working with children in situations that may or may not endanger them emotionally or psychologically. All needs to be done legally and with proper documentation.