This is helpful, and I hope these other platforms grow in popularity. However, my concern with kids is that they will desperately want to use the platforms that their friends are on and they will hold it against the parents (and alternative platforms) if they are forced to make do without the big tech ones.

I think addressing that will be helpful. What I would add:

• Talk about alternative front ends and teach kids about them. Its possible to access the big tech sites without the ads and tracking, and often its a much better experience. You could also explore other ways of using the platforms with limited permissions, such as by using the mobile browser instead of the app, and/or custom extensions that modify the platform (ex. uBlock origin removes ads). This way, kids can still see some of the content that their friends see (under parental supervision), and they can talk about it with them / participate in the group dynamic. They might even feel superior for knowing how to get around the problems that their friends complain about.
• Work with other parents to transition on to these other platforms. If the kid and their close friends are on the better platform, then all of the stuff above is a moot point :)

edit: by alternative front ends, I mean something like Redlib for Reddit: https://redlib.catsarch.com/r/aww/

There is a list here: https://github.com/mendel5/alternative-front-ends