What does the law allow a widow regarding her home, if her husband leaves that home to his children from a previous marriage?

In Florida, a husband could not do this.  The Florida Constitution has a homestead provision that gives the surviving spouse a "life estate" in the house and the right to live in it as long as she lives, no matter what the will says.  Most states give spouses a right to "take against the will" or "spousal rights" no matter what the will says.

I suggest that you consult with an estate lawyer in your state to find out what your rights are based on your specific circumstances.  If your husband has passed away, you definitely need to consult a lawyer immediately.