The first must see is the Dynamikum. It is not just for kids, everyone in the family can have fun in this hands on museum. It has all displays and exhibits in German, French and English. Many of their staff also speak English. http://www.dynamikum.de/en/
I also think the WAWI Chocolate Factory is worth a short visit. It is mainly a factory store but you can also see how chocolate items are made (we went today and saw chocolate Easter Bunnies made).
http://www.wawi-schokolade.de/
