Why must a mother's dna match that of her biological children? why would damian expect half of the children's dna to come from their mother? why not 30% of it? or 70% of the dna?

Because that's how it normally goes

Because each parent contributes to their child 23 chromosomes, so therefore each parent gives their child half of the DNA of the child.

It wouldn't be 30% or 70% because the balance of the chromosomes the parents contribute is always equal.