My Coworkers Child (4F) is Constantly Interrupting Me at Work

So first, I want to clarify that I truly adore this little girl. My coworker and her daughter are very sweet, and that is why I am here looking for advice because I don't want to upset either of them. That said, the daughter will constantly come to my desk (my desk is in a showroom and has no door) and want to play, color, or chat even after I have said "I can't do that right now." She will push and try to make compromises like for example: Her "let's play hide and seek", Me "I'm sorry, honey, I cannot play right now I have to work." Her "well let's just do one round for each of us then we can be done" I again reiterate that I cannot play, and she will keep pushing compromises. I eventually cave because if I make her upset, I will feel horrible and like coworker and gm are judging me. Yes, my GM knows and does nothing (coworker is her assistant) except get onto me when things are not done saying I should "just tell her no". My supervisor has been stepping in because she is an older woman and the little girl I think is intimidated by her, but she isn't here as much as I am. I feel it's not my responsibility to parent someone else's kid, but coworker most of the time won't interfere because she is getting her own work done without interruption. I feel like it is typical behavior for a four-year-old, but I don't have kids so I'm not sure how to navigate this. Any help is appreciated, thank you!

Update: I am 27F, sorry I forgot to put that. Ok so, this business is small. Like less than 50 people small. We don't have an HR. My department is also a relatively new addition to the business, with less than 30 people on our team.
The girl is allowed bc 4 years ago my coworker put in her 2 weeks because she was close to delivery (totally get that). Owner and gm wanted to keep her so they let her start working from home. (Very kind of them and super happy for coworker)
She still works from home because it's very close by, and comes over for a little bit at a time to help clients or get work done with gm. They share an office space that isn't far from mine (they have a door) I am toward the front of the building out in an open showroom for construction services so she is never unsupervised.
I would feel less guilty if this coworker and I didn't get along, but I am super grateful to her. She helped me learn a lot when I started my job, and she genuinely is a good person and mom to her kid. This is recent behavior that has been getting excessive. Like id say less than a month. I appreciate everyone's advice. Some truths are hard to hear. I totally understand the criticism and appreciate the validation of thinking SHE needs to keep her daughter with her and explaining respecting people saying no.

I'm going to update this to keep all the info in one place. Again, thank you.
Update 2: so from all your comments (thank you so much it's helped a lot) there are 3 ways to go about this:
1.) be firm, walk her back to mom, and stick to it. I'm going to do this today.
2.) Red and Green light game. Red means no talk or play, green means come say hey! If this is still an issue and mom and management won't do anything. This is second option. Not ideal bc there is a reason I don't have kids myself (I don't want to parent) but this allows me to keep the peace without being impolite.
3.) last resort: find another job. I'm always looking for better opportunities, but this is recent behavior and I do like my job besides this issue. So this is really just if all else fails.
I really do appreciate the comments and advice, even if it's impolite, it's what I would probably say if I was straight forward about the situation. I'll update with what happens and what works!
Final Update: hey everyone so this is probably my final update unless something crazy happens. I was firm! I just said, "hello [child's name], I am very busy today so I will not be able to play." She said something like "you said that yesterday". I stood firm and reiterated I could not play. She went to her mom and mom actually kept her in her office. I didn't have to say anything again. I think something happened at home that triggered the change in routine.
At the end of the day, I thanked Mom for understanding that I was very busy and that even though I felt bad for having to say no that I really did have a ton of work. She was very understanding and confided that she is taking steps to teach her to respect when people say no. She encouraged me to say no, and that she would back me up. Overall, I think everything is going to be fine.
Thank you again to everyone who commented and is following along. Sorry it wasn't a "dramatic" ending, but I'm just glad it was a good one.
What makes the whole situation so annoying is the distribution of inconvenience. Mom is in a closed office getting her work done in peace. The GM is offering the extremely helpful advice of just say no from a safe distance. The person actually dealing with it is sitting in an open showroom with no door, no backup, and a growing collection of guilt about letting down a child she genuinely likes. The kid is not the problem. The setup is the problem. Everyone who could fix it has decided it is someone else's job.
The guilt around saying no to a four-year-old is real and also completely manufactured by a situation nobody designed intentionally. It feels mean because she is small and sweet and just wants to play. But caving every time is not kindness, it is just conflict avoidance dressed up as warmth, and it teaches the kid that no is an opening bid.
One firm afternoon, and it was over. Mom was already handling it at home. The kid adjusted immediately. No drama, no fallout, no job search required. Three weeks of guilt and hide and seek, and it turned out all it needed was one consistent no and a parent who was actually on board the whole time. That information was available much earlier and nobody thought to mention it.
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