The design has not been CE-marked, which means it cannot be sold.
Mr Murray says their focus has been on keeping the arm open-source and freely available.
While the device is not as robust as traditional prostheses, which can cost thousands of pounds, children grow quickly and regularly require new ones anyway so they do not necessarily need a long lifespan.
Glyn Heath, senior lecturer in prosthetics and orthotics at Salford University, said 3D printing would not work for all prosthetics, especially lower limbs.
"The materials you can use are limited and the properties of those materials are not necessarily appropriate for taking heavy weights," he told the BBC.
The cost of traditional prosthetics was high because of the long process of getting them to market, he added.
"It takes a long time to develop prosthetics - they have to be tested to specific medical standards," he said.
"Not a lot of people use upper limb prosthetics, so the costs of manufacturing are also high relative to the amount you are producing."