Therefore, the idea that one might want to carefully orchestrate the child's language learning experiences in the early years or to seek a professional opinion if the parent was concerned about language development was not very popular. Watching children develop competance in two languages is highly rewarding and opens many opportunities to them as they grow. Best wishes to you as you think about how to accomplish this in your family. Some of this does matter when making your choices. They were raised in Japan since the age of 4 and 1 respectively.
Some children can develop two strong languages from the very beginning.
The important thing is to make sure that verbal cognitive growth can proceed smoothly. If a child doesn't develop a good vocabulary in at least one language it becomes difficult to learn to express complex concepts. We didn't listen to the warning signs because we assumed her difficulty was due to growing up bilingually since 1 year of age and that it would all sort itself out eventually. This exposes the child to much vocabulary that isn't necessarily part of daily conversation. When the child is reading fairly fluently in that language then add the second.