Why do we use l´ in front of enfant, but not in other cases such as Gousse d'ail (a garlic pod)

Bonjour Anish !

This is a very interesting question, and one many people are struggling with :)

The difference comes down to the specificity of "ownership". Let's take two similar examples:
1- la chambre de l'enfant  versus  2- la chambre d'enfant

1- The room belongs to a specific child = the child's room
2- The "child" element is characterising the type of room, but doesn't indicate who specifically owns that room  = the nursery / "the child room"

I hope that's helpful!
À bientôt !