The two girls were "touched inappropriately" during a visit to the attraction, in Windsor, Berkshire, last Thursday, police said.
A 22-year-man, from Bracknell, has been arrested on suspicion of sexual assault and is currently in custody.
Thames Valley Police previously described the offences as "extremely rare".
Detectives have appealed for any witnesses to come forward.
The assaults happened between 12:15 BST and 12:30 at Castaway Camp in the Pirate Shores area of the attraction, the force said.
A Legoland spokesperson said the parks' management were "deeply shocked" by the incident.

Shares of JP Morgan, Citigroup and Wells Fargo reported a mixed bag of second quarter results, putting pressure on share prices.
The sentiment spread to other bank stocks, leading to decreases at Bank of America and Morgan Stanley.
Gains elsewhere offset the losses, as investors shrugged off weak sales data.
The Dow Jones climbed 0.4% to 21,637.74 and the S&P 500 index rose 0.47% to 2,459.29. The Nasdaq gained 0.6% to 6,311.41.
US retail sales fell 0.2% from May to June - the second month in a row they have declined, according to US Census data. However, May's decline was revised to a fall of 0.1% from the initial estimate of a 0.3% drop.
The sales drop did not appear to hurt retail stocks. Walmart shares were among the market's biggest winners, rising 1.7%.
Stocks were also boosted by weak inflation data, which could reduce the possibility of a third increase in interest rates by the Federal Reserve this year.
The Labor Department said the Consumer Price Index (CPI) rose 1.6% in June from a year earlier, down from a rate of 1.9% in May. The core CPI measure - which ignores food and energy costs - was unchanged at 1.7%.
The S&P 500 and the Dow Jones are up 9% so far this year, while the Nasdaq is ahead 16%.

Clement "Butch" Desmier, 68, was repeatedly stabbed with a knife and screwdriver at his home in August 2012.
David Lawler, 33, and 20-year-old Nathan Jefferson - who turned 17 on the day of the killing - ransacked Mr Desmier's home as they hunted for cash.
At Bradford Crown Court, Lawler was told he would serve at least 16 years, while Jefferson must serve 34 years.
During the trial the jury was told how Lawler, from Shipley, had been given a 10-year extended jail sentence for slashing a man's throat with a Stanley knife nine days after Mr Desmier was murdered.
The jury heard that more than 60 injuries were inflicted on the pensioner, whose face was covered with a cushion during the attack at his home in Rowlestone Rise, Greengates.
Lawler - described as the "dominant personality" of the pair - denied murder and witness intimidation but was convicted last week.
Jefferson pleaded guilty to murder.
The police inquiry into the case lasted more than three years and included undercover work and a Crimewatch reconstruction of the attack.
A man has been arrested on suspicion of sexually assaulting two six-year-olds at the Legoland theme park.
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US stock indexes closed higher on Friday despite declines in banking shares as investors reacted to the latest round of results in the sector.
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Two men have been given life sentences for murdering a pensioner in a brutal stabbing attack in Bradford.