Hi, I'm new to the site, but saw your posting re CAFCASS. My dh and his ex wife had contested residency of my ss, so if your x is contesting it then you may have the round of interviews we had. First they saw dh and his x separately, then saw ss (aged 12) and interviewed him and showed him flashcards. from this they wrote a report recommending that my dh had residency. his ex wife threw a wobbly, and insisted CAFCASS officer came to see her at home with her son, which he did, but didn't change his view.
I think it depends on how old the child is, what their wishes are if they're a certain age, and whether there is any conflict about care/shared care.
For us it was a nightmare, but I don't think you have anything to worry about over an interview. Just be honest, as they are really trying to do what's best for the child, and if you have concerns that are genuine, then tell them.
Although the result for us has been mixed - I have my own step parenting problems - it's important that the parents focus on the child and not scoring points off each other, which is what my dh's ex tried to do.
Hope this helps xx Good luck xx