Having watched the video my first thoughts were why can't (for eg) the child's grandma reply using her email client. Why does grandma have to login to the website?
I presume one reason (as suggested in the video) is that Maily wants to promote "kid-friendly" email replies. I imagine kids (particularly the very young) would find it more fun to receive a visual reply than a standard email reply.
Re: Maily: Your Kids' First Email
#51An account is still a barrier. They could create a "click to read and respond" link that contains an access token.