If you have a very small child, you might consider a bag for your car seat. You can rent car seats in Germany, but the kids always seem happier in their own. If your child is old enough for a seat, but still young enough to need a car seat, you can take it onto the plane and use it there. Be aware that a big bulky car seat can be a royal pain to drag through the airport and on to the plane.
The stroller, if you need one, should be small and easily folded. Trying to get one of those fancy jogging strollers into your rental car, along with your luggage might be difficult.
Germany has a moderate climate. Warm in the summer, and cold in the winter. Be aware that you could face rain in July though, so bring a raincoat or an umbrella. Pack comfortable clothing if you plan to tour a bit. And play clothes for the kids to romp around in. Do bring a light jacket for evenings (or rain).
Don’t forget to pack comfortable shoes. Flip-flops are only worn to swimming pools, wearing around the city is not comfortable at all.
All baby and toddler items can be easily found in stores and aren’t too expensive, so you don’t need to bring your own if you don't want to!