I see absolutely nothing wrong with it as long as both children can be supported fully and comfortably. If this woman is going to end up living paycheck to paycheck then it's not a good idea. If she has the means to comfortably live with two young children then go for it.
I'm Gunna go against and say no. I have two kids from a marriage that my ex wwalked off and left. They have never had or known a dad. I have raised to know that not all kids have dads etc... it still bugs and hurts them on days at school that are about dads or at Parks with dads. It didn't so much when they were little but as they get older it does. Not all families are the same but consider all aspects before doing.so