I agree with you that Cameron showed an appalling lack of empathy here, when you might have expected him to feel some, both in his treatment of the NHS and for carers and disability benefits. However, the situations are not comparable, as the Camerons have the financial security to never need to rely on the NHS or benefits if they don’t want to (though they did claim benefits they didn’t need and then promptly whipped the rug out from others who did need them).
If Starmer has a trans child, this is very different, as they won’t be able to opt out of society, they will be subject to the laws and policies same as everyone else.
It is relevant for voters to know if Starmer is conflicted here, in the same way we wouldn’t allow a judge to preside over a legal case he or she had a vested interest in. Similarly, doctors can’t make medical decisions for family members. The conflicts of interest are obvious.
Moreover, you can’t expect voters not to want to know the truth of the situation when it is difficult otherwise to fathom quite why Starmer has been in approval of unregulated life-alterating medical experiments on children, and in favour of stripping women of the right to equality in the workplace and sports, and happy to put women at risk in prisons and changing rooms.