during a divorce hearing, if the child is asked up to the stand, is he/she legally obligated to testify?

You can be compelled to testify. The judge would have the right to not have you testify or limit your testimony if he/she deems your testimony is not relevant, or is purely (and unnecessarily) duplicative of other evidence, or would be more harmful to you and it is worth to the case (though this would typically only be in the case of minor children, who may be emotionally harmed by the experience, not adults, who are assumed to be able to deal with the circumstances), but that's at the judge's option: there is no blanket or general exception from having to testify against one's parents. If you believe that there are valid grounds to not testify, you can file a motion to not do so (e.g. motion to quash a subpoena), but will have to support it with facts showing why the testimony is irrelevant, unnecessary, and/or harmful and legal citations (e.g. caselaw) to the same effect. Or if one parent is looking to have you testify against the other, that parent can make the appropriate motion for you. A lawyer would be helpful in doing this.
You can only take the 5th against answers that would incriminate you of a crime--the Fifth Amendment protects you from incriminating yourself of a criminal act. You cannot take it to protect yourself from non-criminal liabilty or consequences, or to protect another person.