Nerd you're in Qld right? I would persist, no it's not compulsory, but a child starting off grade one who hasn't been to prep will, in all honestly, be way behind the 8 ball. I know our prep teacher well and have discussed the new curriculum with her - prep is basically now grade 1 standard. If a child missed that whole year, to catch up in year 1 would be a big ask. I don't know why it isn't compulsory to be honest with the teaching that is now occurring.
A is well ahead already. He's a smart cookie. In all honesty I don't think he would be behind academically, it would just be a matter of learning how to be away from mum again.