This video mentions a study which showed that speaking 2 languages on a daily basis can help delay the onset of Alzheimer and dementia. The act of juggling the 2 languages turns your brain into a muscle and allows you to process information better and more efficiently, as well as storing more information.
In this video, Brenda K. Gorman (PhD in Communication Sciences and Disorders with a specialization in language and literacy development of bilingual children.) discusses four common myths associated with bilingualism:
1. It causes children to be delayed
2. It is confusing
3. Bilingual children are less smart
4. Speaking English only gives better chances of success in life
And she concludes by saying that it is better for parents to communicate with their children in the language they speak best, in order to provide them with richer grammatical and vocabulary models.
The whole interview is very interesting as it shows some of the reasons why minority/heritage languages, for example, are suppressed by English or other majority languages. But I thought that the 4th myth, the one about having better chances of success is the most relevant in this case because she argues that reducing to one language, in this case English, can cause negative emotional and social impacts within the family, as the child may feel excluded.