Play centre chiefs said Oliver Kennedy, who is 3ft 3in (100cm), was too tall to be classed as a toddler when he visited the Jolly Roger Adventure Play Centre in Swindon, Wiltshire.
But they defended their decision by saying Oliver was a health and safety risk and they were simply trying to make the centre as safe as possible for all children.
Mum Amanda Sparks, 24, was told that because of her son's height she would have to pay a full child price of £3.60 instead of the toddler admission fee of £2.20, which she normally pays every week.
Her friend's daughter, who is one month younger than Oliver, was able to enter as a toddler - despite the fact Oliver will not reach his third birthday for another two months.
Dad Dylan, 30, told the Swindon Advertiser: "Ollie is tall for his age but, because he is asthmatic, he has had to take steroids from a young age which has probably made him grow a bit quicker.
"To me it just seems like complete discrimination against my son for being tall. "He is only two so he is definitely still a toddler. I would say you probably stop being a toddler at about four years old, when you start school.
"They go there almost every week and he even had his second birthday party there last year – Amanda went to pay the normal toddler price and was told because Ollie came up to the line she would have to pay more.
"She paid it because she didn't want Ollie to know she was disappointed because he was so excited. But when they queried it after with the manager they were told 'If you have a tall child then it is just tough'."
Dylan emailed the management team the same day and was told that the decision had been made due to health and safety reasons.
It's not really about the price, it is more about the attitude that we have been given," he said.
"When they measured him he just came right up to the line, but that was because his hair was all spiked up anyway.
"It just seems unfair when my friend's daughter, who is just a month younger than Oliver, is classed as a toddler.
"He can still play in the toddler area once he is inside because that is done on age, but the price is done on height, which doesn't seem fair to me."
Jolly Roger manager John Bolton told the newspaper: "We do go by height at Jolly Roger – a bigger child would wear the equipment out quicker than a smaller child would.
"But if an older child wants to play with their younger brother or sister in the toddler area then we wouldn't stop this, as we want the children to be happy and play together, so we can mix and match.
"We would never stop friends playing together because of their height – that is not what we are about.
"We try to be as accommodating as we can and make the play areas as safe as possible."
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