Wikipedia:Simple talk/Archive 96 WikiKids - Vikidia
What I pointed out above is based on experience I collected when editing this Wikipedia. As to the UN declaration, it is a very nice try, but as most UN declarations, its use is probably limited. Children are still developing, they do have at least one legal guardian. The model of any encyclopedia is that those people supplying the content are sufficiently knowledgeable in their field of knowledge. This means that you will also face the issue of motivating these people to do so. Not using the vocabulary means you sacrifice precision and you need editors that understand the subject matter very well, so they can simplify it to a degree understandable by your audience.