Thank you for the feedback everyone. My list of suggestions is far from complete, though. I forgot a really important one:
When they - as these transgender ideologues inevitably do - bring up people with differences in sex development (DSDs, aka intersex people), you could ask the following question:
Are you aware that the Scottish Government recently apologised to people with DSDs for conflating their issue with trans issues?
They said they should never have conflated the two and promised not do this again after intersex advocates and intersex rights organisations protested about the Scottish Government lumping the two groups together as if their issues had anything in common. They also find the appropriation of the language people with DSDs have created to describe their trauma (sex assigned at birth) deeply offensive.
(On the treatment of children alone, their interests are diametrically opposed:
Intersex rights orgs are campaigning for as little medical intervention as possible, as late as possible for children.
Trans rights orgs are campaigning for as much medical intervention as possible, as early as possible for children.
The former argue that children cannot give informed consent until they are adults who can truly understand the longterm consequences, the latter argue that informed consent can be given by children young enough to believe in Santa Claus.)
People with DSDs are only ever brought up as a gotcha to argue that sex is not dimorphic and immutable. While those intersex people exist who do reassign their sex because it was indeed wrongly assigned at birth, statistically speaking, for the UK this amounts to one person every two years.
So you might ask them to please not appropriate the complex and often traumatic medical issues and language of another vulnerable and marginalised group to raise awareness of an unrelated group whose trans status and related issues we are always told are not medical conditions, after all.