This is the 13220th most frequent Russian word.
многодетным
Having many children.
'многодетной' serves as an adjective modifying 'семье', describing it as having many children.
Он вырос в многодетной семье.
He grew up in a large family.
'многодетных' acts as an adjectival modifier in the genitive plural form, referring to families with many children.
Существуют специальные пособия для многодетных семей.
There are special benefits for large families.
'многодетной' is an adjective in the instrumental singular form, modifying 'матери' to indicate a mother with many children.