If like you said, she really has no understanding of money and budgeting as opposed to just wanting to spend her salary on crap, I'd do the following.
Sit her down with your/your OH's wages in front of you (if you have a partner and they're in work). You may not want to do this but my parents were always really open with us about how much money was coming in roughly, so a quick calculation would give us the answer to 'is what I'm asking for completely unaffordable'.
Have a spreadsheet ready that lists all the household bills including water, electric, WiFi, council tax, insurance etc.
How much is rent/mortgage. Explain about the interest on a mortgage.
Do you have any cars? Paying off any credit cards? Same as above.
Now you've done a rough calculation of how much it costs to run your household vs how much money comes in each month, before you even get to food/clothes/treats/birthday and Christmas presents.
Ask her what she thinks would happen if the fridge broke down, your boiler needed replacing or a pet gets ill. Emergency money talk here. Unless you can't afford to have an emergency pot, in which case tell her how lucky she would be to have one.
A lot of children are growing up 'protected' and we're told they shouldn't know what's going on financially in the house. I think they absolutely should. In an age-appropriate manner, but if you grow up understanding how finances and budgeting works, you'll know how to handle money in the future. She needs a wake up call, it will have huge benefits for her in the future to learn about these things.