Your counsel has already fallen on deaf ears and caused a row, so I wouldn't risk going for round two and getting fired.
You are their employee. That means that while you are 'on their clock' so to speak, you are bound by your employers' rules. If they deemed it OK for their child to enjoy endless hours of screen time, then you need to abide by that, whether you agree with it or not.
So the way I see it, you have several options.... Mind your tongue, Look the other way. Keep your job. You can also maintain above options if you decide to search quietly for a new job. If and when you get hired and as you prepare to walk out the door, consider talking some sense into those fools one last time, blazing row or not!