"Most" of the ones I use support it, it is an rfc on its way to becoming a standard, but if you prefer I say "some" then ok, some email accounts support it. I think older versions of Exchange do not support it but outlook.com does support it so probably newer versions of Exchange do support it. SmarterMail supports it, Yahoo does not seem to support it.
Each mojoportal user account must have a unique email but if the email is not functional then it only means the user will not receive emails. Things like password recovery, newsletter, webstore order notification and forum notifications, rely on email but may not matter in your scenario. If they do matter then it would be best for your users to have functional email addresses. I have no other recommendations. I would say use the sub addressing if the parent has an email that supports it or ask them to create an email for their children if you want to be sure that mail can be sent.