Set your entertainment budget for the household at whatever you are paying now for streaming services and when they inevitably want to add Paramount or Discovery onto the what is available in household, you say “that is fine, but we can only have 3 streaming services (or if the kids are older: we only have a budget of $x month for streaming), which means we have to cancel one in order to add this new one”
Because it’s hard to claw back what they already are used to, you’re putting them in a position where they must consider opportunity cost. If they are clever, they will say, “let’s switch for 3 months so we can watch what we want and switch back to our original setup.” This way introduces scarcity without being the “bad guy” - after all, the budget is the budget. It also gives them choice but not the freedom to choose everything so they can take some agency and better prepares them for the challenges of the Real World(tm).