nirogu

If the child is really young, check out the sugar desktop environment. There is an official distro from sugarlabs and there is also a fedora spin (fedora soas)
If the computer should be a little more functional, the GNOME desktop or the Deepin desktop are good options imo

As far as I know, no. But you can from mastodon, pleroma, or other similar microblogging oriented software

Although the most active instance by far is pixelfed.social, that shouldn't be relevant as you can interact with any pixelfed or mastodon account/posts no matter the server you join. So, you could join any pixelfed instance and follow the mastodon.art (most active multimedia instance) users, for example