Purchase controls for TV, movies, apps, etc, which all actually work, screen time controls, time-of-day control, web site limits, etc. Along with reporting functionality and integration with parents' phones for ad hoc approvals and time extensions.
I haven't found anything on Android that allowed me age-appropriate fine-grain controls that didn't require exceptions like turning the entire rule off just to allow a single purchase, or having the same functionality controlled in two or three different overlapping spots, etc.
If you really want to see how bad it can possibly get, take a peek at Amazon's Fire Tablet parental controls. Those take the cake for opaque settings that sometimes do what they say, sometimes much more, sometimes nothing.
The most infuriating thing about Android/Google Play from this perspective is that there's no way to prevent a child from installing literally any and every free app which has the lowest rating.
On iOS you can disable installing apps, uninstalling apps, updating apps or disable the app store completely using Screen Time and before that you could (and still can) require a password/fingerprint/face ID for any app install including free.
It would take almost no effort from Google's part to replicate the last of these (add an option requiring a password to install any app, even free with lowest rating) but they seem to be unwilling to do so.
That only works if you set up a separate child account for a separate device and then manage it using Family Link.
If I want to hand my own phone over to a child for a while, there's no way (that I've found anyway) to prevent a child from installing all sorts of crapware on it.
On non-ancient Android versions you can have multiple user accounts. Create a new one and log it into your kid's family linked account. You can switch the user before giving it to them and they'll get the expected restrictions.
I haven't found anything on Android that allowed me age-appropriate fine-grain controls that didn't require exceptions like turning the entire rule off just to allow a single purchase, or having the same functionality controlled in two or three different overlapping spots, etc.
If you really want to see how bad it can possibly get, take a peek at Amazon's Fire Tablet parental controls. Those take the cake for opaque settings that sometimes do what they say, sometimes much more, sometimes nothing.