Don't believe a sabot or optimist is going to fit or suit the 2 of you - but see the last link below RPAYC use optimists and it appears pairs of kids are in them ok. This first link discusses starting ages for Manly Juniors (sailors are in the 8-13 years range) but this would be based on a skipper around the top of that range and the crew nearer the lower end.
manly16s.com.au/manly-juniors/#:~:text=What%20Junior%20classes%20of%20boats,up%20into%20the%20Flying%2011.The Manly Junior should fit the 2 of you and be the ideal 2 person training boat to get a 6 years old into the sport without scaring him off. Unless you are totally familiar already suggest visit the Manly 16s and view the boats and the typical skippers and crews. From there you both could go to Flying 11s if you want to, or he would go into a sabot or optimist on his own or (for the sabot) with another youngster. See good summary for sabots here;
www.narrabeenlakessailingclub.com/index.php/sailing/sabot-classRPAYC only have Optimists;
rpayc.com.au/courses/kids-learn-to-sail/Suggest measure the bow of your cruiser to confirm fit.