Why do you think more kids feel they want to change sex? My guesses are chemicals in the food chain & water supply. Plastics in the body maybe.
Testosterone is lowering and male genitals becoming smaller. Homosexuality occurs in the animal kingdom but does Trans also?
My view would be that in regarding to children there's a large element of social contagion, especially amongst girls, just as there is with anorexia and self-harming. Much of it fuelled by the Internet.
The numbers for adults in their 40s and 50s being referred to gender clinics are more stable, I think. There's certainly no big upsurge in middle aged women coming out as men, which you might expect if it was only due to increasing social acceptance of trans people.
I also think there is much more pressure on young people to conform to hyper sexualised and hyper feminine/masculine stereotypes than there was 30 or 40 years ago. Kids who don't fit these stereotypes are encouraged to think there is something different about them, when it's perfectly normal.
Toys and clothes are far more gender specific these days. The media promotes hyper femininity as the ideal, eg Kim Khardashian. We really are back at that point where kids who play with toys aimed at the opposite sex are now singled out as trans, whereas 40 years ago they would be called gay.
Staff at the Tavistock apparently joked that there would be no gay kids left - it's one reason many gay adults are against medicalising children, they see it as 'transing away the gay' and homophobic and regressive.
I don't think it's related to plastics etc personally. That wouldn't account for the largest number of referrals being young people, and being girls.