Newborn bath?

Hi! Just had my baby on the 11th and she had a diaper rash yesterday that is pretty much gone now after I put corn starch on it.

My mom wants to bathe my daughter (4 days old) but only from the butt down and away from umbilical chord. I’m not entirely ok with it but I trust my mom. Fiancé is uncomfortable with it and wants to do sponge bath only.

Any suggestions/advice? Would it be okay to do the bath with being careful about not wetting the umbilical chord?