The Children's Commissioner for Wales has criticised a UKIP AM for making a "personal attack" on Swedish climate change activist Greta Thunberg.
Sally Holland said she was "mortified" by Neil Hamilton's tweet that included a picture mocking the 16-year-old.
Ms Holland has written to him expressing "disappointment" that Mr Hamilton could make an "attack on a child on social media in this manner".
But he said it would be preposterous if Ms Thunberg was immune from criticism.
Ms Thunberg, whose solo protest outside the Swedish Parliament inspired the school climate strike movement, has been lauded for her emotive speeches to politicians.
The UKIP AM's tweet included a picture of Ms Thunberg which has been treated to give her demonic eyes.
Mr Hamilton said in the tweet: "It's shaping up to be the hottest day of the year.
"Please remember: no electric fans. DEFINITELY no air conditioning.
"Greta has spoken.
"Have you done anything this week that Greta might disapprove of?"
Responding on Twitter, Sally Holland said: "I am mortified that a politician in Wales could make a personal attack on a child on social media in this manner.
"I have written to Mr Hamilton today expressing my disappointment in his failure to uphold high standards of public conduct."
On Tuesday, the teen activist lashed out at French lawmakers for mocking her in a speech to parliament that was boycotted by far-right politicians.
In her speech, Ms Thunberg responded to her critics and restated her demands for urgent action from governments to curb carbon emissions.
She said children like her have become "the bad guys" for daring to tell politicians "uncomfortable things" about climate change.
Reacting to Ms Holland's criticism, Mr Hamilton, who is AM for the Mid and West Wales region, said Greta Thunberg had "very publicly entered the adult realm of political controversy".
"If cabinet ministers such as Michael Gove take the opinions of this 16-year-old seriously then public criticism of her should be fair game," he said.
"Miss Thunberg is currently pushing for policies estimated to cost the UK taxpayer £1 trillion which will hit the poorest the hardest."
The UKIP AM said the "Greta-inspired 'Extinction Rebellion'" had also been responsible for nationwide disruption and the notion she "should be immune from criticism after all this is preposterous".