Hmm.
I think it might be a good idea to take some lead from the other lawyer here, but the terms of reference outline exactly what it is that you want the expert witness to assess.
Generally, you want the expert witness to either prove or disprove any issues in the parent/child relationship that may have been raised or denied by the other party. If sexual abuse is an issue, for example, you want the expert witness to assess the children for potential evidence of same, whether they fear the accused parent, etc.
Think about what you need the expert witness to provide evidence on, then go from there.