Brachychiton are easy from seeds. I use Debco seed raising mix (has heaps of sand in it) and coir seed raising mix 50/50 that you buy in a block and soak in water. I use this mix for all my seed raising.
I use coir as it helps to retain moisture with our extreme dry temps we have in summer. It also has fertilizer in it which gives the seedlings a good start.
Coir is made from coconut husk. It does have peat on the label and this is what puts a lot of people off using it. It is great stuff and have been using it for 12months now in most of my soil mixes.
Copied this from Wikipedia: In horticulture, coir is recommended as substitute for peat because it is free of bacteria and fungal spores, and is sustainably produced without the environmental damage caused by peat mining.