Why not just provide a Mandarin class at our preschool? From a marketing perspective that might make sense due to the fact many parents are happy to find out a school is offering foreign language classes. However, it is not best practice. Adults who took foreign language classes are living proof! They often can say some introductory salutations and find the bathroom and that is about it (even for the adults who took several years of language in high school and college- https://www.youtube.com/watch?v=kO8x8eoU3L4).
A Mandarin class exposes a student to another language, but does not engage the child in that language. Mandarin immersion creates a similar environment as if the child actually would study abroad. A child who is immersed in a foreign language environment for 4-7 years develops native level fluency. So just think about it, your child could be fully fluent in two languages by the time they are in elementary school. Why not give your child that gift?