Even if it's not CASA regulations, or even airline policy, in my mind it is quite revolting for anyone to wander around without shoes, at any age. It's not hard to put a pair of shoes on a child when you leave the house. It's civilised, and good on the FA for doing something about it, irrespective of any rules.
Personally I think even thongs, sandals and any other open shoes should be banned from aircraft. 1. Because I don't want to smell other people's stinky feet, and 2. There is clearly a safety issue. Sure, shoes might not help you survive a crash (as some have suggested) but they might just be what you need to survive on the ground afterwards.