The six-year-old girls were assaulted by a man at a pirate-themed playground in the theme park during the visit on August 11.
The two families had gone to the resort in Windsor together during the summer holiday, as the youngsters had it on their “bucket list” of things to do.
They were assaulted between 12.15 and 12.30pm while playing on a fort structure in the Castaway Camp, reports the Press Association.
Police are to make an appeal on the BBC’s Crimewatch programme in a bid to catch their attacker.
One of the mothers told the programme: “I wasn’t concerned about them being in there, I was concerned with one of them running out and getting lost, so Laura was at the entrance to catch them.
A reconstruction of the incident will be broadcast on BBC One at 9pm, followed by the police appeal.
In August, Thames Valley Police said: “Such offences are extremely rare and we are working closely with Legoland to identify the suspect.”
The offender is described as a white man, possibly under 5ft 8ins tall, wearing dark-coloured slim-fit trousers, trainers and a dark-coloured T-shirt.
He was not known to the victims.