But her room is a safety hazard
If I had to get to her at night in an emergency, I would kill myself. Same goes for if she had to get out in a hurry in the middle of the night.
That's a reality of life. And it can be approached in that way.
The way most parents solve problems is they identify the problem and then come up with a solution. Then they drop the original problem and turn to the new problem of "How can I get the kids to comply with my solution?"
But if the original problem is focused on, then we can ask the kids for help solving the problem. How can we keep this room so that I can come to you in the middle of the night in the dark without harming myself? And how can you get out without harming yourself?
Their solution probably won't be to keep the room spotless! But it is their room and the problem isn't to keep it clean. It's to remove the safety hazard.
The child may come up with a solution but maybe finds that it isn't easy to do on a regular basis. So rather than turning the problem into how to enforce the child's solution, look at the solution and figure out what's not working and talk about other solutions, always focusing on the original problem, not turning the solution into a problem.
And it's modeling problem solving for them: analyzing a problem, synthesizing a solution, evaluating a solution and revising are skills that are useful for a lifetime and applicable to every problem a child will encounter in the future.
But I wonder .... sometimes clean up isn't about what mom wants or what her standards are, sometimes it's about safety of younger members of the house.
Safety at least has a reason behind it that kids can understand. If there are no toddlers, mom insisting that art must be cleaned up "because we're supposed to" isn't really understandable in terms of "must". It is just the way mom wants it. What will happen if it isn't cleaned up? The house will be messy? And?
But if there are toddlers, mom saying to the older kids in an informational tone -- not disguised as a command -- "If the scissors are left out, Little Dimitri might find them and slice up the curtains. Would you put them in the drawer for me, please?" or "If the markers are left out Little Annika might color all over your trading cards. Could you put them in the box in the cupboard, please?" is perfectly understandable. They might still forget! But at least they understand the reasoning behind it. There are real understandable potential consequences to not cleaning up.