Yes telling them to observe the correct Islamic hijab is obligatory upon you and it it is part of enjoining the good and forbidding the evil.
Telling them this should be done in the correct manner and not in a way which is threatening or controlling.
Regarding telling them what to wear, if you mean by telling them not to wear the clothing which is not islamically approved then then is fine.
O you who believe! save yourselves and your families from a fire whose fuel is men and stones; over it are angels stern and strong, they do not disobey Allah in what He commands them, and do as they are commanded. - Quran 66:6
May Allah grant you success