If their friends are on Snapchat I’ll suggest that you try to bring Signal into that friend group.

How old are they?

The minimum age for Snapchat is 13. The direct mesaaging part of it allows for fun and harmless messages between friends. I think allowing her to use it if she's 13+ is reasonable.

What you could do is to ask her not to allow friend requests or messages from anyone who are not her friends, not to use the AI assistant (which can't be disabled), and to not use it to consume content from influencers (which is in a separate tab to the right). You cannot really police this, but it's not the end of the world if she sees the feed of some dumb influencer. Maybe while she's home you can show interest and use it together with her.

Position data must of course also be disabled. Snapchat is a bit creepy.

The fediverse is always (somewhat) public, making it not inherently child friendly. Getting together with other parents to set up a Pixelfed instance to use in the friend group rather than to have them use Instagram is a cool idea, and allows parents to be admins and decide who can federate without taking control of the accounts of the kids. It might be a nice way for them to learn that whatever they do online is run by some person whom they decide to trust. And it could keep them off Instagram a little longer.

I don't really know the first thing about parenting though, just my thoughts. It's a tricky question.

When do you think you should allow a child to have an email address? A Fediverse account is basically an email account, except that the primary inbox is shared with a lot of people rather than only one person (and the same goes for any social media account). If you wouldn't allow someone to use an email address, you probably shouldn't allow them to use a Fediverse account either.

At least one person who seems interested in the health of children expressed that "delaying children’s access to smartphones until high school and social media platforms until 16" is a good idea. https://www.anxiousgeneration.com/ https://jonathanhaidt.com/social-media/ https://www.youtube.com/watch?v=V0MXgA2sSn8

matrix synapse good learning expierance on how the internet works and is more closed then snapchat so less risks

Not the most related, but if Instagram becomes an issue, there is an Instagram patched app which is DMs only

For real-time communication, the most suitable solutions are probably documented around https://www.privacyguides.org/en/real-time-communication/ (note that email is not recommended for person-to-person conversations).