The cast is made up of children from the British Theatre Academy’s Aylesbury group and some who took part in open auditions.
There are three teams of young people - two based in Aylesbury and one based in High Wycombe meaning the lead roles will be shared.
Among those sharing some of the main roles are Aylesbury sisters Isabel and Zoe Forward.
Isabel, 14, is sharing the role of Ariel the mermaid while Zoe, 12, is sharing the role of Ursula, the evil sea witch.
They are joined in the show by their younger brother, Isaac, eight, who forms part of the ensemble.
Their mum Catherine said: “They have both been rehearsing since the first week of September.
“This week they have moved to the Waterside Theatre and have been rehearsing there.
“The British Theatre Academy is great because it gives children and young people the chance to get into theatre - and there is the chance to go on and perform in the West End.”
The family live in Wendover Road and both sisters also starred in Annie, the BTA’s first foray into Aylesbury.
Catherine added: “The girls have been practising together at home and testing each other on their lines.
“They are not in the same show because they are in different teams but it is great that all three of mine are involved.”
Show director Dean Johnson said: “The BTA believes that all children and young adults should be able to train and perform without the barriers of cost and offer auditions and repertory theatre all over the country.”
There are two performances a day of The Little Mermaid Junior at the Second Space in the Waterside from Saturday October 21 to Wednesday October 25.
The weekend performances are now sold out but tickets are still available for the shows on Monday and Wednesday - starting at 5pm and 8pm - tickets start from £18.15.