There are two possibilities. If you are using it as a noun it is "two-year-old". How many children do you have?" "Two. A four-year-old and a two-year-old." The other way is if two years is used as a description, telling how old the child is. "How old is he?" "He is two years old." [Notice no hyphen] If I was talking about multiple (let's say three) children who are two years old and using the noun form it would be: "Three two-year-olds"