A lot of initial language knowledge is transferred through the parents, especially at an early age. So the more words you know and use, the more your child will learn as well. Get a lexicon for children (like: a real lexicon, not just a book with pictures and words beside them.) and explore it together - the explanations will lead you to new words you do not know yet (and extend your child's knowledge about the world in general).
A thesaurus can help you to discover new meanings of words you already know or other ways to say the same thing. Employ this newly unlocked knowledge in your conversations.
You can also use your regular shopping trips to learn a lot of everyday vocabulary: What is in all these cans and packages? Can you find Küchenpapier? Which way do I have to go to get there? How do you call this long stick with brushes at the end? Since everything is labelled, you always have a solution at hand.
Read, write, talk and listen as much as you both can. New words one has encountered over the day is a valid dinner conversation (and sometimes even more interesting then all this grown-up talk).
Play with words. Especially since German allows compound words, you can use it to create new ones or take them apart to figure out what they might mean.
Also sometimes switching, adding or removing a single letter can yield unexpected results and as far as my experience goes, young children enjoy these "nonsense conversations" a lot:
Krankenkasse (health insurance)
kranke Kasse (sick register/budget)
Krankasse (a register/budget for/from/of? cranes)
Kranklasse (a class about cranes/for crane driving)
Krakenklasse (a class full of octopi)
klasse Kraken (awesome octopi)
Krakenkranklassenkasse! (A budget for lessons to teach octopi driving cranes)
And finally, be patient and don't despair. Formal language, bureaucratic language, common language, slang, dialects and trade specifics can make for an overwhelming mix even native speakers struggle with from time to time. Encourage your child (and yourself) to ask what a specific word means or how something is called. The truth is, that sometimes Germans don't know either.