There is no rush to respond. Let it settle in your mind and take a week or so to figure it out.
One possible response, just to get the conversation started:
Dear Ex,
The court has invited CHILD to attend our upcoming hearing. As you are probably aware, he/she is happy to attend and contribute as appropriate. An outcome of the hearing will be the court's decision on advisory reports or expert evidence necessary to inform any outstanding determinations.
My understanding is that this process should he no more invasive for our child than is absolutely necessary. As a result, I suggest we await the court's permission.
This approach will ensure that any assessment is not contested, does not have to be repeated, and is directed as the court deems appropriate/necessary.
Yours...
EDIT
www.justice.gov.uk
The requirement for the court’s permission
5.1
The general rule in family proceedings is that the court’s permission is required to put expert evidence (in any form) before the court (see section 13(5) of the 2014 Act for children proceedings and FPR 25.4(2) for other family proceedings.). The court is under a duty to restrict expert evidence to that which in the opinion of the court is necessary to assist the court to resolve the proceedings. The overriding objective in FPR1.1 applies when the court is exercising this duty. In children proceedings, the court’s permission is required to instruct an expert and for a child to be medically or psychiatrically examined or otherwise assessed for the purposes of the provision of expert evidence in the proceedings section 13(1) and (3) of the 2014 Act.