Do kindergarten level child need a Student Visa to attend kindergarten here?

Children below the school going age (that is 7 years old) are not required to apply for a Student Visa. They need only apply for a Social Visit Visa.

If my children are already married and want to study in Malaysia with their husband, do they need to pay the fixed deposit?

Not necessary. They can apply for a Student Visa once they have obtained places of study in Malaysia.