Slower to learn languages: Even though they are better at multi tasking, the study also found that bilinguals are slower to acquire vocabulary than are monolinguals, because bilinguals must divide their time between two languages while monolinguals focus on only one
Classes in most schools are taught in only one language. For the child to become fully fluent in reading and writing in a second language, there will be additional work for the child to learn the second language. The time spent could take away from other activities and studies, which could be a major trade off
Often, children who can achieve native fluency in a second language grow up in an environment where the parents need to be actively involved to teach the child the second language