Hi Hendo,
the usual advice is to get lessons. I would add to look for a good instructor who is used to working with kids.
We wrote a comprehensive blog post about kids learning to kitesurf here:
www.kitebud.com.au/how-young-can-you-start-kitesurfing/As explained in the article, for kids, it would be best to have a kids seat harness (yes, they make some for kids like the Prolimit Rookie Seat Harness) combined with a bar with short throw that also has a mini chicken loop (Airush Access bar is an example). The combination of these 3 things (seat harness, short throw bar, mini chicken loop) make the bar much easier to reach for kids.
A 6m for Perth would make a lot of sense @50kg. There is not much need to get a twin tip bigger than 135cm. At this weight they don't need a big board and a board too big would be too hard to handle, heavy and difficult to control/edge.
Hope this helps
Christian