First, don't worry if your children between the ages of 4-7 do not know Spanish. They will pick it up after a few weeks. Older children will take longer. We have a son (now 5-1/2) who just completed Kindergarden here. Our son was fully conversant in Spanish after only a few weeks of school. He now speaks perfect Spanish with a perfect porteño accent and we have only been here for seven months. He starts first grade in March. We were very pleased with the school and plan to continue with the same school for first grade. Every private school advertises themselves as "bilingüe/bilingual." The school my son attends spends half the day in English and the other half in Spanish. However, none of the teachers is a native speaker of English which is a concern of mine. When looking for a school, see how they teach English. Are they merely giving the kids a few words here and there, or are they speaking to them and teaching in English? I think this is important along with the native speaker issue for teachers. I don't want my son, born in Ohio, to speak English with an Argentine accent because he learned from Argentine teachers. Anyway, these are a few things to look for.