Think of my children is just another step along the line of pearl clutching. But I’ll do you one better, I had a covert video taken of me and circulated around the internet and school that I went to. Would e2e encryption made any difference to that? No.
A parent wouldn't let an 11 year old go to a frat party. They wouldn't let them go unsupervised to a concert. Most probably wouldn't leave their children home alone for a week. So why in the fuck do their kids have unfettered Internet access? Do they also let the kid just go to anyone's house without at least knowing who that friend is?
How would unencrypted messages preventing grooming of your niece? Her parents should be checking on her devices the same as they'd want to know who her friends were and where she was when not at home. A phone or Facebook is not a babysitter and certainly not a parent.
If E2EE was banned or restricted what at all would have changed with your niece? If she was groomed on completely unencrypted IRC what would be different? If she was groomed by some friend's older relative what would be different? Her parents failed at several occasions to actually parent her. Restrictions or banning E2EE wouldn't make them more capable parents.
What changed is the message she send will be unencrypted and not E2E, so now more people have access to them and maybe save or send them to their own closed messaging platform.
And confounding this issue is the fact that the major platforms (Facebook, et al.) simply don't want parents to have this kind of control over their children, because it will effect their bottom line. Children with less adult supervision tend to be far more easily manipulated.
Its not just child diddlers who want this manipulation to proceed un-impeded by parental oversight - its Facebook, et al. too!
So this evil campaign is going to get a lot of corporate support, I fear. For the wrong damn reasons.
(Disclaimer: am an angry parent who is sick of the shit my kids have to deal with online... but who will also NEVER give up personal encryption as a right.)