For iPhone, see this discussion: https://discussions.apple.com/thread/8232191
This may provide ideas too: https://protectyoungeyes.com/vpn-explained-virtual-private-network-parent-guide/
Maybe you need to go old fashioned: take the devices away and ground the kid.
There are many apps for iOS(and I imagine Android too) that let kids bypass parental controls. Even on a PC or Mac, they can use proxies to bypass parental and school controls and access "forbidden" websites. As soon as the parent blocks something, the kid finds a way to get around it. This is why I think maybe old-fashioned parenting measures may need to be deployed (such as confiscate all devices, ground the kids, and other consequences).
I remember the thread a while back concerning the Parental controls, but that was more blocking sites than setting times. I also remember that it wasn't working for me until I disabled IPv6 in my computer, as the IPv6 was automatically bypassing the HT2000W's control, as its controls, or at least with website blocking, only pertains to IPv4.
Ohhh..... *those* sketchy things. I forgot they even existed. There's one I was directed to many years ago to make it look like I was from the UK so I could watch BBC stuff online.
As soon as the parent blocks something, the kid finds a way to get around it.
And you just know that, when something's blocked, it becomes something kids MUST see, and they'll do anything they can to.
I remember when I was six or seven years old my mother had locked her bedroom door, which she didn't normally do. Well, that piqued my interest, of course. Needless to say, I was able to get in, and lo and behold, I found out that Santa had come early. At that point I discovered how it was that Santa was able to deliver so many toys during one night. He didn't. He delivered for a long time beforehand, and the parents were in on the secret. That sneaky Santa!