It ain’t the ‘90s and this ain’t Windows 95 with bypassable-by-accident OS account logins and half-assed website blocking made by the lowest bidder. Getting past app installation restrictions and time limits on iOS would be… challenging.
It sucks as a parent because you get this from both ends: “parent better! (Putting in tons of work that our parents didn’t have to)” and also “lol what are you doing restricting kids on computers is impossible, give up you idiot”.
(And on some platforms it is, for practical purposes, impossible—looking at you, Linux, not just because it’s a powerful open platform but because its permissions and capabilities system is decades behind the state-of-the-art and tools for sensibly managing any of that on a scale smaller than “fleet of servers” and in the context of user-session applications are nonexistent)