@RedBeech there's a lot of confusion between boundaries and control I think.
A boundary in your example would be, "I don't leave my children with people who let them watch CBeebies" and who then, if MIL lets them watch cbeebies, looks after them themselves instead. That is, in my opinion, a ridiculous boundary, but fine, if you want to choose to pay for childcare or look after your own children all the time to ensure they 100% do what you want 100% of the time, fine.
Control is "I won't let you see my children and I will punish you or I will shout and scream at you because you didn't respect my boundary"
Sadly, so many people think the latter is a boundary. And you're totally right, it's baffling.
We did not allow BIL to be with our children without one of us present at all times as he was a drug addict. This did not mean he was not allowed to go to MIL's house if our DC were there. It meant that if he was there, me or DH had to be there too if we wanted our DC to be there. That was our boundary but we didn't expect anyone else to change their behaviour to accomodate it.