How do I restrict internet access to specific hours in iOS 14?
I have found a solution! It's not perfect, but I've been testing it and it just about works:

In Chrome, on the device in question, go to this address: chrome://flags/
Then search for &quot;screentime&quot; (all one word).
You should see: Enables ScreenTime Integration
Switch this to ENABLED.

It's a bit glitchy, but it works. It locks Chrome in the way you'd expect it to.
In the meantime, tell the developers how important this is to you here: https://bugs.chromium.org/p/chromium/issues/detail?id=1162695
It seems the longterm plan might be for Chrome to fully support Screen Time (like Safari does), allowing users to manually allow access to specific websites during Downtime.
More information: https://www.chromestory.com/2020/09/chrome-ios-screen-time/
It's unclear when this feature will be added however.