Hi Reformer,
Here is a well designed resource with several versions of the catechism, including one for younger children. I purchased two of them so that children could read with the parent or teacher.
http://www.amazon.com/The-Family-Worship-Book-Devotions/dp/1857924010/ref=sr_1_2?ie=UTF8&qid=1375534437&sr=8-2&keywords=Family+worship
On a personal note, regarding the catechism, I found it fruitful to review it line by line with my children one on one, so I could focus on their personal understanding of the tenets. We viewed the catechism as a project, taking time once a week with each individual for about an hour over the course of about a year. We got in the habit of having our time together on this project at a small local pizza restaurant, to help frame the catechism not as a feat of memory strength, but as a cherished memory.
Blessings,
MC