It's not so much a matter of "she's roping you in to be a beta male provider" theory crap...(she might be, you don't know).
It's the lack of respect for everyone involved. Many women try this whole, "me and my kids" are a package thing. I've had that one thrown on me a fair bit. And those women I don't date. Why? I'll tell you.
Number one. It shows a complete lack of respect and understand for the minds of her children. What sort of an example are you teaching them? How can you teach them it's okay for mommy to keep bringing strange men around on a regular basis? What message does it send about their father?
Two. It disrespects their father. Yes, it does. A man deserves to know his children are being protected and cared for, bringing strange men around who could be anything (drug dealer, pedofile, rapist, crackpot mental patient) is not safe and shows you don't care about your childrens wellbeing, nor the opinon of their father who surely wouldn't want their kids around strange men.
Three. It shows her character. She is only concerned with what she wants. She's not willing to make sacrifices for her children's sake. What sacrifices? Well, getting a sitter, seeing you only when she is free. She wants what she wants. And also it may show that she is looking for a provider, telling you upfront what she expects from you in regards to her children. Or it shows she's just young and stupid and hasn't had the home training/ thought process to understand that this is a bad idea.
Lastly, it disrespects you as a potential mate/date/suitor. You already know she has children. If you want to go out with her, you want to get to know her. Eventually if things get serious you should be allowed to get to know her children as well. SHe is asking you to get to know 3 people at the same time, she is asking you who is not a parent to immediately decide if you want to deal with the same burdens she is saddle with, when you don't even know who she is. She is asking you to deal with her children's personalities when you may not even have children and you don't know her well enough to decide if you even want to deal with her kids. It's presumptious. It's putting the cart before the horse. It's not a good idea in my opinion.
I would tell her this, "I want to take you out, give you a break to be an adult for a while and to have some fun, I want us to take some time and get to know each other first and if things go well then we can bring the kids along once we know each other better". If she won't go for that. Drop her. Honestly...drop her anyway. 21 and two kids...yeeesh!