The biggest concern with loose substrates is impaction from ingesting too much while eating. Calcium sand is also bad because it is basically Tums so it lowers the acidity of the stomach causing issues with proper digestion, as well as clumping more and having a higher chance of impaction. Babies are more susceptible to impaction due to smaller digestive tracts and poor accuracy when eating. If you won't be feeding them on the substrate, it isn't as much of an issue. I would steer clear until your dragon is about 10-12 inches long. A 70/30 mix of organic topsoil free from fertilizers and washed play sand is good, and you can add some excavator clay to help it hold burrows better if you can find it but it isn't necessary. The Australian sand is good but a lot more expensive. Topsoil/sand is perfectly fine and cheaper.