RPG Maker is totally fine. I started with RPG Maker when I was young in the 90's. There is a way of editing the assets slightly, if I am not mistaken, so you can probably create something rather unique and more original this way. Default assets may slightly affect the originality score, but you can make up for this with Fun / Engagement and Community Spirit. I love the goal of making a game along with your son.
I look forward to see what you guys turn out! Good Luck!