Louise Kadayer's comments come after the NSPCC said the number of counselling sessions it was giving to children, who were unhappy or confused about their gender, had doubled in the past three years to a record number.
Last year the NSPCC held 2,796 counselling sessions in 2015/16 - an average of eight per day - with children who felt their biological sex was wrong.
That's more than twice the amount in 2012/13, when 1,102 sessions were held.
Kadayer said for some children talking therapies were only ingraining gender identity issues further rather than solving them, and that other therapies should be considered instead.
She also said there's been a "really scary" increase in the number of children medically changing sexes and then regretting the change and going back. She argued only adults should be allowed to make lasting changes to their gender.
Other stats:
In the last three years counsellors held more than 260 sessions on gender with 11-year-olds.
The most common age range for calls was from those aged from 12 to 15.
Homophobic bullying, which included transphobic abuse, was mentioned in 450 of the sessions.
In 2014 a survey found that 48% of trans people under 26 said they had attempted suicide.
Speaking on Premier's News Hour, Louise Kadayer said: "Talking therapies for older children and teens... in lots of cases it actually has a detrimental effect, so much so that in other countries such Australia, talking therapies for children just don't exist.
"My problem is the increase in really drastic interventions children are having in terms of things like hormone replacement, stopping the development of bodies. Now once you do that, there's no going back.
"We've also seen a really scary increase in younger people having this dramatic life change and then needing it reversed back."
A Government spokeswoman said: "Children should be able to be themselves and all agencies have a role to play in supporting them as they discover who they are.
"That's why the NHS is investing an extra £2.2 million in gender identity services for young people, alongside an additional £250m a year for children and young people's mental health services - and we are investing £2.8 million to help tackle homophobic, biphobic and transphobic bullying in schools."
Listen to Premier's Alex Williams talking to Louise Kadayer on the News Hour:
