I am o negative and so is my ex husband we have 2 children together my little boy is o positive but my little girl is o negative is that possible please really need an answer because my ex isn't claiming her because his wife says there's no way she's his because of her blood type and I never cheated on him but he has me

No,

Rh negative children can have parents that are either Rh positive or Rh negative.

If both parents are Rh negative then the children will be Rh negative.

However, a child whose parents are both Rh+ can be Rh -.

You can read all about it here.

ABO Blood Types Inherited from Our Parents - BloodBook, Blood Information for Life