it depends.

In community property state, 401K accumulated during a marriage belongs to both parties. Better find out u state law.

Coming to the time of divorce, make sure u do the best to protect u child's benefits, both emotionally and materially.
Also, being a single mom does not mean you carry the child rearing job alone. Even though he is no longer u hubby, he is still father of your children. Give him the chance or responsibility of be a father physically, emotionally, and financially.

Best wish :)