M.M.
Start speaking German to them. Especially things like Let's go!! Dinner!
Bath time.
Get books in English, like Go Dog Go and translate while you read to him.
Do it with all his little books until you can get a hold of German books.
Say a German grace before dinner.
Just start.
I have a German student in the house and she learned English and Italian this way. She can actually old a conversation in Italian after only having it for two years. THe teacher stands up in front and speaks in Italian and repeats herself a lot. She does not use German.
Start with little things.
Meine Tshirt ist gelb. Meine hosen ist blau. Heute ist ______. Danke, Bitte
The more you make an effort the more they will learn it.
Good luck.