According to Brown’s 12 principles, the Direct Method would be the most appropriate way to teach children an L2.
Since children absorb language, and their LAD is at its peak, if we introduce them to a language “violently”, without giving any previous notion of the target language to them, the will not notice any difference between learning their L1 and learning an L2. Moreover, children learn more efficiently by discovery, they are always analyzing things and, since the Direct Method teaching is inductive, they themselves would prefer this method. Vocabulary is taught through demonstration and association of ideas: meaningful learning (which is better because there is a grater chance it will be remembered). There is little (or none) focus in the teacher; he or she tries to adapt him/herself to the way children learn better and this, according to the BP is the best way to teach. The better time children spend learning, the more they will remember in the future. If they receive positive strokes constantly, they will feel self-confident and they will be as comfortable as in their mother tongue, while talking in their L2. They will not feel any fear when taking a risk, this way, the results will normally be positive. Their native language will not interfere at all in learning an L2 because they will be able to express themselves almost the same clear through the two languages; therefore, there is a great chance to establish their performance at the same level as their competence.
One of the “disadvantages” of this method is that not every teacher would be able to apply it. Teachers must be native-speakers or have to have a very high level of the language in order that they do not need to say any single word in the children’s L1 (which may be his or her as well). Another “problematic” point would be the number of children per class. Classes must be much smaller than the ones we are used to and this is expensive.
I do not think countries like Spain, which spend very few money on education, are still prepared to apply the Direct Method in a broad way; therefore, until these countries evolve, we will just have to resign ourselves to having this kind of teaching in private centers or schools.