Knowing vocabulary is the basis to being able to read, speak and understand the language, so you'll want to teach new vocabulary as often as possible. Have you thought about using games to introduce and review vocabulary words? They work great with children because instead of simply memorizing a word, the games actually help them USE the word in context... it's a more meaningful experience than just memorizing the meaning.
I teach new vocabulary in every lesson. Then in the next lesson I always revise what has been done before. With young 4 to 5 year olds I might only introduce one or two new words in a given theme, and continue to work on the vocabulary previously introduced, plus the language structures. With 6-12 year olds you can introduce more each time.
For example I might introduce 6 new words in the first lesson and a sentence structure. In the second lesson I'll revise the 6 words, add 6 more – or less depending on how the class are doing, and incorporate another target structure into the class.
You can use games to revise large amounts of vocabulary in a few minutes and that is a good way to start the lesson. If you can include regular short but powerful bouts of revision into your classes your pupils will do so much better. Normally teachers rely on students to be motivated enough to revise things at home, so if you can make it a part of your teaching your class will do better overall. With vocabulary it is crucial – we only remember what we use – so if they learn some words in December and never see them again, well they certainly won't be able to produce them in June!