I started a spartan company and tried to invite my son to join but he is unable to because he has a child account. How do I change this so that he can join? I changed his birthday on his Microsoft account and made sure all things would allow it in privacy.
You should be able to promote his account up to a full account, Microsoft detailed out how to do this on their support page here. You’ll only see the option though, if your son meets the right age for his region, you can check that here. Let me know if that works for you 
I’m still having this same issue and the link provided does not work anymore.
Please advise. Thanks.