Compound nouns are made of two nouns put together like girl-friend, travel agent and so on.
When we make plural of such words we add ‘s’ or ‘es’ in the end:
Girl friends, travel agents, house wives…

However, when first noun is ‘man’ or ‘woman’ then these nouns are also made plural:
women drivers, men drivers…

Also in compounds where noun is followed by preposition and another noun i.e noun+ preposition+ noun, then first noun takes the ‘s’ or ‘es’
mothers-in-law, brothers-in-law, wards of court…