I am currently a very eager learner of German, so of course I wish that such a thing existed. But I agree with Angela that the existence of simple English wikipedia isn't really a firm argument for the creation of the same thing in German, for all the reasons that she gave.
(Additionally, it is very likely that the demand by end users for a simple English wikipedia is much higher than the demand for simple German, simply because so many more people are learning English than German at any given moment in time.)
I also think it's important to keep very clear that simple German for adult language learners is a very very different thing from German for children. The problems faced by the two groups are quite different.
The original poster was interested primarily in a wikipedia in German for children. This raises a whole bunch of very interesting questions, I think, and we should not confuse those questions with the questions of simple language for adult learners.
I have no advice on whether such a thing should be pursued or where or how. :-)
--Jibo