It would fall under the accusative form, however, since "Tochter" is plural, then changes in the pronoun ending only apply if the case of the noun is masculine. All endings with other cases remain the same.
I am struggling with breaking sentences down into nominative, accusative and dative (not reached genitive yet). Therefore I am really struggling with questions like these. Are there any good links to other sites that can help me please?
Yes, "mag" can be used with either first or third person. Its a "modal" verb (although not being used as a modal in this sentence), so the third person present doesn't have the usual suffix. (And the plural has a different vowel, and let's not think about other tenses.)