which one is correct?

Hello! I would like to communicate in my cv that I used to look after 2 kids. One was 2 the other one was 5 years old. Which one is the best way to do it in one sentence and which one is correct?
I used to look after a 2 and a 5 years old child.
I used to look after a 2 and a 5 years old children.
I used to look after a 2 and a 5-year-old.
I used to look after a 2-year-old and a 5-year-old.
Thanks you very much!