The performers are aged from 10 to 17. "There is so much talent here".
The title role of Shrek will be played by 17-year-old John Paul College student Justyce Moala.
He said auditioning was a last-minute decision, when one of his friends told him to go along and try out.
Justyce was originally cast for a smaller role but was swapped to play Shrek, and when he found out he was shocked and overwhelmed.
Justyce said he started taking drama seriously in Year 12, which was when he found he had a passion for it.
He said he enjoyed the adrenaline he got when he went on stage and was looking forward to opening night.
"Knowing your lines is a challenge. You can't let down your cast and have to be top of your game with lines and attendance."
Ms Benfell said they were looking for a cast of about 40 and 100 children auditioned.
They ended up casting about 60 to include as many youth as possible.
"It's cool working with all kids. It's a completely different experience, I'm really enjoying it."
This would be the first production she had directed with just children, though the theatre had run a junior youth programme called Ratbag Theatre.
She said junior shows were great vehicles for young people and gave them a chance to be part of something bigger.
Ms Benfell said she was looking forward to watching the young people take ownership of their performances.
"They think on a completely different wavelength to adults.
"That's one of the most rewarding things for me doing this production and seeing the kids get out of their comfort zone."
There will be 12 performances with four matinees.
The Details:
What: Shrek The Musical Jr
When: September 1 to September 16
Where: Casablanca TheatreTickets: $15 children, students and seniors, $20 for adults, family ticket of two adults and two children $60. Tickets available from Ticketmaster or Sir Howard Morrison Performing Arts Centre in early August.