You can get unlimited Data on many plans for about $50 a month and just turn your phone into a hot spot. it doenst cost $100 a month.
The problem is you keep asking a Kasher of a Maaser, Whenever I bring up some clearly Unshabbosdick activity you can you will shut off the router or the 4G costs too much. Maybe the kid is stealing the Wifi from a neighbor. Maybe a bunch of kids share Netflix. What ever he did , he got Netflix to work on his iPad (Kids arent stupid, they probably know more than you and can defeat any roadblock you put in front)
Maybe the kid has a job to pay for things like their phone (Dont make a Kasher of a Maaser what kind of job and you can stop him from working)
I never advocated doing it in the living room, Obviosuly you tell the kid to go upstairs and watch in their room quietly