In my opinion, it is very subjective and can be different from person to person. Some people have ability to learn many languages at the same time, which is called polyglot. But for other people they can only learn 2 or 3 languages at maximum, because they don't have natural ability to be a polyglot. Genuine passion in learning as many as languages is also a huge factor to determine how successful someone to be proficient in many languages. As I haven't had children, I'm trying to picture myself if I have kids. I would see first whether my kids have natural ability to learn multiple languages and observe how big their passions in learning new language. Let's say if my kids speak Indonesian or English fluently as their mother tongue, then I'd add one more language as their second language, for example French. I would rather keep them focusing on their French instead of letting them learning multiple languages at the same time. This way they can speed up their progresses to master their French. However, this is a pure subjective opinion in my point of view. Everyone has different thought and I respect that.