Well, if it's a parent saying it to their own child, they could rephrase it as:
I'm YOUR adult so you'll do as I say.
or (for the pedants)
I'm your adult and what I'm asking you to do is not illegal, immoral, harmful, neglectful, sexually, physically or emotionally abusive, racist, zenophobic, triggering, sexist, transphobic, heteronormative, disablist or discriminatory against any protected characteristics. Therefore you'll do what I say.
But I reckons "I'm an adult so you'll do what I say" is quicker.