I have found a way round this which you can do on your phone, should work directly on child’s phone, but as I have created a family on my iCloud i can control it from mine instead.
Follow the steps below- I’ve added photos as it was easier to explain.
SETTINGS
SCREEN TIME

click on your child’s name

See all activity

in “MOST USED” click on chrome


you should then be able to select ADD LIMIT


I set an hour, he can have more time if I enter the screen time passcode. I suppose you could do this and then keep giving them more time as and when. I found this to be the only way to stop my son spending hours on chrome after his downtime started as screentime locked the other apps on his phone but not this one. He was still going on Instagram but through chrome.