Can I use a cable splitter to send data dn image on 2 phones at the same time, both phone use USB type C, My son would fly will I check all the camera setup for a proper video/photos.

I did not find anything on a quick search for using 2 devices (master/slave) for working with the Mini.
You could try it and see, but in the back of my little seive of a bwains, tells me there could be signal conflicts with 2 devices connected at the same time, "talking" to the controller.

Could you just use one phone, with a long cable?  You don't need the phone to be locked into the controllers arms to be able to fly the drone. If you see what I mean?

Hi. Thank you for reaching out to DJI Forum. With regards to this query. I hope that our valued DJI co pilots can provide you the best response for this inquiry. Just a friendly reminder that DJI does recommend using cable splitter this may affect the performance of the DJI Fly application and the said drone during flight. Thank you.

Thank you all for your response, I guess I will have to setup the camera, then let him fly.