B.C.
Aw, come on now!
Computer/phone/device use is a privilege - not a right.
The best parental control is:
PARENTS TAKING AWAY THE ELECTRONICS!
Set a time (an hour or two) of recreational computer / device time (non school related).
They must use these things in public areas of the house where everyone can see what they are doing and with who - never in the bedroom(s).
When the time is up - the computers/devices get powered down and put away.
If anyone sneaks using anything, lock them (the devices, not the kids) up in your room when they are not suppose to be in use.
A locking file cabinet comes in handy for this purpose.
If they are doing something inappropriate by your definition (no one elses definition matters), then you say
"No - we're not doing this and if you do this anymore your electronics recreation time will go away (for however long you want to ground them for)".
That's what setting boundaries/standards/rules is all about.