If my husband dies, am I entitled to part of his 401K?

You are certainly enetitled to the portion of the 401k that accumulated during your marriage but his child may be entitled to the portion accumulated before your marriage.  Since his child is the named beneficiary, that child can immediately claim the benefits so you ought to make sure the plan administrator knows you have a claim as well.  The administrator will hopefully hold back from distributing the proceeds to your husband's child until the conflicting claims are worked out.  You ought to retain an attorney since there are important legal issues to address with regard to your husband's naming a beneficiary.