Wikiversity will provide you with resources to allow you to teach your children foreign languages, in addition to having a strong command of their native language. This will be done in a clear and concise manner. Seeing that Wikiversity is still a young project, please help by contributing.
What is a multilingual kid? 
A multilingual child is a child with ability in more than one language, that is to say, thatt they have a native language (L1) and any number of additional languages (L2). Modern linguists make no distinction between the number of languages a person speaks in addition to their native language, regardless of how many additional languages they speak they are bilingual. A language learner's skill ranges from the very basic, to "near native speaker"(NNS) and "native speaker"(NS).
Why would I want my kid to be a multilingual one? 
Simply, because knowledge is power. Giving the ability to your kid to communicate in one or more language beside her/his native language is a boost to their life overall, hence learning new languages opens a vast space to diverse cultures.