What content children get exposed to has always been the responsibility of parents. Assuming this bill has honest intentions (I don't believe it does) the issue here is more that parents are giving their children unrestricted access to the internet.
What we need to be doing (if anything) is giving parents more ability to regulate what content their kids have access to rather than simply putting that responsibility on the government and service providers.
ISPs should be making it much easier to for parents to add global content filters and device-specific filters. Both could easily be managed from an app and in my opinion this should be what the government should be focused on.
That said, what this seems to actually be is an attempt to crack down on content and speech the government (and perhaps the public generally) doesn't like.
The UK is an extremely authoritarian place, not because people here are generally authoritarian in nature in that we seek to control the behaviours of others, but because we're excessively conformist and polite. This bill exploits this aspect of our nature because few people in the UK are going to stick their necks out in defence online encryption or adult content.
For context for those who live the in US, in the UK if you're doing something impolite you're generally breaking the law...
Swearing in public? That's a public order offence. Saying something mean online? That's hate speech. Protesting fossil fuels? That's disruptive. And so on...