Hey buddy this doesn’t look like a great service. It isn’t open source but claims to be, and appears to have only been around a couple of months.

A project with a lot of backing called Nextcloud could work for you. It has a service called Nextcloud Talk that you’ll be able to throw on their dumb phone and if you want to you can use it as an alternative to dropbox/onedrive and google docs.

edit: a benefit of nextcloud talk is that your kid won’t be able to add some random person or do anything too nefarious with it and if they did you’d be able to see everything