That’s pretty much the definition of the job of parent. To control everything around the child and how they interact with things.

The fuck. You'll breed a country of people with zero social skills, zero independence, and a lot of ressentiment for their parents for boxing them in and helicoptering leading to an authority neurosis.

In short, you'll have American conditions.

It takes a village and all that.

For one thing, don’t give kids a smartphone until they’re at least 13, they have no need for one before then.

No, give them 30 pence so they can find a telephone booth and call you if something is up. Make sure to isolate them from their peers because they can't use the same chat app as everyone else. The more isolation the more you control them which will make nurturing that neurosis even easier.

After 13 or there abouts, they are given more freedom and more responsibility to go along with it, and hopefully have been raised well enough to respect that.

If, at the age of 13/14 thereabouts they haven't learned to evaluate things for themselves, have had the opportunity to make wrong choices that they then learned from, they'll be rolled over by puberty hormones driving their frontal cortex to mindless exploration. You cannot substitute your own judgement for theirs, your judgement isn't stopping them, their capacity and ability to say "wait a minute I should think before I act" is the only thing that can.

From there, limitations and guide rails will remain in place, be it a traditional curfew in the evening, or a limitation of “screen time”, and if course of what the children interact with online.

At the age of 16 they should be mature enough to live on their own, with parental backup being present, but not imposing on them. They'll call you when they need help because they came to value your guidance. Not control. One of the two begets rebellion, the other doesn't.

Eventually you have to let go, let them be adults and make their own decisions,

I'm sure you'll be able to after helicoptering them for 18 fucking years and them going zero contact for their own sanity.

but all you can do as a parent is try to prepare them

Then fucking do that!