How very difficult for you.
I'm a stepmum in a past similar position, and I can't stress how important it is she feels your children are an extension of you.
If she doesn't feel this she'll have trouble bonding with them and accepting them as part of her immediate family.
If you marry they will become her children as much as ones born to you both.
She sounds as though she doesn't grasp the enormity of the situation. She seems to be doing what she thinks is enough without realising it takes a 'mother' to welcome children.
Do you have them staying frequently? Does she get time with them on her own to bond? Does she appreciate that she's not just getting you, she's getting a ready made family? These are things I feel she's not grasped.
In her head maybe she's viewing your relationship with her as separate from your relationship with the children.
It's not the same start to a relationship when children are in the mix. It's very different as you have precious cargo to protect. She's not ever going to get what she would have had from a man without children. If she expects this she's in the wrong place.
It's early days yet. What is important is that these children feel that your home is their home too. That you have their best interests at heart. That they are loved and welcomed. Not with presents, but with time. These are their formative years.
Your children are so young, which is a good thing as you have time to get this right. It also means they are vulnerable too.
I wish you well. You may have some tough conversations ahead.