"Their father is strict."
Translation:Pater eorum est severus.
"eorum" is the genitive plural masculine (or neuter) form of the pronoun is, ea, id ("he, she, it"). -orum is the gen. pl. masc/neuter 2nd decl. ending. Translate it as "their" or "of them".
All the genitives plural have some kind of "m" ending: -arum, -orum, -um/-ium, -uum, -erum (that's for decl. 1-5 respectively).