The girls were "touched inappropriately" in the Castaway Camp playground at the park in Windsor, Berkshire, on Thursday 11 August.
Thames Valley Police refused to give the suspect's age or say where he was arrested, citing "safeguarding issues".
The boy has since been released on police bail until 2 December.
Police previously issued an E-fit of a suspect, who was described as a white man in his teens or early twenties.
Officers said he was white, under 5ft 8in tall, wearing dark slim-fitting trousers, a dark t-shirt and trainers.
Police began following up new leads in September after a reconstruction was shown on BBC One's Crimewatch.
A 22-year-old man previously arrested on suspicion of carrying out the attacks was released without charge.
Thames Valley Police said: "This is remains a live investigation and we continue to appeal for the public's help".

Both 48-hour and 24-hour strikes have already been held on seven Shell-operated platforms.
Further action was planned but was put on hold for fresh talks.
Wood Group and the Unite and RMT unions said in a statement they had held two full days of constructive discussions which would continue next week.
The statement said: "These collaborative discussions have demonstrated a clear, shared understanding of the issues being raised by all parties, plus the challenges facing the North Sea.
"We remain focused on reaching a mutually-acceptable outcome, which demonstrates collective leadership in shaping the future of the North Sea."
The initial 24-hour strike on 26 July was the first industrial action of its kind in the North Sea in nearly 30 years and was followed by a 48-hour stoppage the following week.
The platforms involved are the Curlew, Brent Alpha, Brent Bravo, Brent Charlie, Nelson, Gannet and Shearwater.
Aberdeen-based Wood Group provides maintenance and construction to Shell and signed a three-year extension to its contract earlier this year.

However the Social Democrat (SPO) Christian Kern said he could not work with any groups that "incite against people and minorities".
He was sworn in on Tuesday, replacing Werner Faymann who quit after losing the support of party colleagues.
The SPO were trounced in the first round of presidential elections last month by the Freedom Party.
The vote was won by the far-right party's candidate Norbert Hofer, who faces a run-off against the Greens' Alexander Van der Bellen for the mostly ceremonial role.
It was the first time since World War Two that candidates from Austria's two main parties, the centre-left SPO and the centre-right People's Party, did not make it to the run-off.
Mr Hofer has suggested that if elected he could use his power to dismiss the government of the day under certain circumstances.
At a news conference, Mr Kern said working with the Freedom Party was possible if certain criteria were met, without specifying what they might be.
He added: "My plan is not to lead the SPO into opposition, the opposite is true.
"But at the end of the day we need an identity and for us it is absolutely unimaginable to work with parties who incite against people and minorities."
Swearing in Mr Kern at a ceremony in Vienna, outgoing President Heinz Fischer said: "You are taking on a great and beautiful but also difficult role with a lot of responsibilities."
A boy has been arrested in connection with the sexual assault of two six-year-old girls during a trip to Legoland.
[NEXT_CONCEPT]
"Constructive" talks have been held between unions and the Wood Group in an ongoing dispute involving offshore workers.
[NEXT_CONCEPT]
Austria's new chancellor has refused to rule out co-operation with the country's far-right Freedom Party.