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 agree, I was about to grab this for my kid, but getting the dozen or so friends and relatives to create accounts isn't worth it. Very nice looking, but should have the option of just using an email whitelist to send. Then include a one time use reply link in each email so the recipient can reply without having an account.
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.