I am not a parent, but I was a child with accepting and loving parents just like you, from as far back as the 1960's.
When I come to this board, which I do now as someone who is ULTRA longterm postop and happily adjusted, to put something back into the community, I always give thanks for my parents and schools having been so tolerant. (which trust me must have been even more difficult back then)
I had a bad patch in my late teens when a well meaning therapist gave my parents the same advice that you have received from the Portman and Tavistock.
Unable to find anyone to support the contrary view they took the advice and so began the most unhappy period of my life, which only ended when in my mid twenties I had finally transitioned and had SRS.
But still I give thanks that because of them I mostly had the childhood that I wanted and indeed I believe that I have fewer hang ups and regrets because of that.
As you are clearly in the UK I would suggest that you look at the GIRES website
http://www.gires.org.uk/and hopefully you may find some help there.
Finally I would say keep on doing what you are doing. If your child grows out of it, so be it, but let them have that freedom to choose.
Incidentally Dr Domenico DiCeglie - of the Tavistock clinic was at my 30th Birthday party, many years back - which was hosted for me by Dr Reid - who is my very good friend and whom you can see in my avatar. I always thought that DiCeglie was more understanding that that... but sadly there are very few gender shrinks who seem to understand the issue from our perspective.
Far too many of them still think they can cure us or discourage us from being something which I genuinely believe is hard coded into us - even though they don't yet acknowledge it.
Google Dr Vincent Harley and his work on Brain Androgen receptor faults in Transsexual patients if you want some early research findings that point to this.
