The better children understand the Mother tongue speak, the easier it will be for them to learn German later on. Therefore: Take the time to speak with your child in your language.
Talk to your daughter and son while you are doing household chores. Listen and respond when they talk to you. Go outside together - to the playground, to the woods, to a lake, to the zoo. Talk about what's there to see, and in doing so, help the Vocabulary to expand. Look at picture books together and tell stories.
On the playground or in the playgroup your child has the opportunity, together with German speaking children to be. Be happy about the words it learns in German. This gives courage to learn more words.