The pronoun "wasz" means, that the, so-to-say 'owner' is multiple (some children) and the object is singular (their dad).
They are 8 types of pronouns in Polish (see a quite complete list in Polish here or another a bit less complete list in English here). "Wasz" falls into 2-nd category - zaimki dierżawcze (possessive pronouns). and they reveal:
- the number of owners (singular / plural)
- the gender of owner - only in 3-rd person singular
- the number of objects (singular / plural) - except for 3-rd person singular/plural
- the gender of object (singular male, singular female, singular neutral, plural male, pluram female or neutral) - except for 3-rd person singular/plural
See a table below: