I might offer my own comments and insight on this issue. Whilst I was born in Jakarta, my father is from Hong Kong and my mother comes from a small Indonesian island called Bangka, where they speak another language. At home she would speak to me freely in Indonesian and Bangka, two languages I remember up till today. My father would offer me lots of Cantonese ndarin material to watch/learn/play with, and I would have learnt that language as a mother tongue had he actually been at home (he is rarely at home and so I didn't get much exposure to Cantonese).

At the age of 3, I moved to Malaysia, where everyone speaks Mandarin, English and Malay. I had to learn those two at a very young age. I ended up speaking all of them, and I now speak English as though it were my mother tongue (having learnt it at the age of 3, it might as well have been!). 

Did I ever get mixed up between so many languages? No, not in the very least. In fact, from infancy I was already able to distinguish between all the different languages and use them at appropriate times. And I'm no baby genius either - my brother got it right too!

Children learn languages effortlessly, because they start acquiring their mother language by simply mimicing the sentences they hear. My German teacher speaks only German with her son, and her husband, who's Egyptian, speaks only Arabic with him, and at school, he learns English. He's fluent in those three languages and doesn't seem to have any trouble with them. I heard that children are able to learn up to five languages simultaneously. It's better to start as early a possible.

from what I have read, language system does not form thought. At least there is a theory from Sapir, Whorf, adn Vygotsky stating that language is essential for thought. However there are some inadequancies regarding to that theory, one of the objections is the case about persons who are proficient in more than one languages where the languages had been learned in childhood. Imagine if the different languages form the different thoughts (let's suppose that the theory is true), then such the people would have formed more than one systems of thought. They would not be able to think coherently since they already have thoughts separated by their different language system. Those different system would come up with the different ideas and those people would have difficulties in using the knowledge they gained from different languages if it said that thought is supposed to be language specific. 

In fact, from the previous comments, We notice there are lots of example about this case where the children can learn more than two languages in childhood. there seems no difference between those multilinguals and monolinguals in terms of beliefs, values, personality or the way they perceive world and nature.

Not at all, I have friend who have grown up bi- or multilingually and their brains seem just fine.  They usually communicate with one parent in one language and another with the other parent.  This isn't always the case but you get the idea. They are able to use both when the situation calls for it or when they want to switch languages. 

Hi! Really interesting your idea and I feel a bit identified because my boyfriend is from another country and he speaks also another language. However, we don't have a baby yet! But I have to say that my future idea is that my child will learn as many languages as possible!! I don't think it'll be confussing for children to acquire more than one language. Quite the contrary!! Childhood is the best age to learn as many things as possible. I'd like to give my child all oportunities I didn't have 'cause my parents just taught me my mother tongue and no more 'cause they just speak one language and that's quite normal. I assure you that if it's my power my child will speak at least three languages. Languages are very important as well as enriching.