When they are 2. I think learning 2 languages simultaneously would be easier for a child than learning them sequentially. But I'm no expert. I learned Spanish and English as a child. My mom says I spoke them both but I don't remember ever differentiating between the two until I got to school. Since then my Spanish has fallen off. I can understand it well but it's a chore recalling how to speak it or learning new words as an adult.
Right now. It seems our ability to learn a second language is diminished by 5 years old. While we are still very capable, we get "set" in our language and have to learn how to pronounce the sounds of other dialects.
right now I learned russian at that age and I thought that I knew it my whole life...turns out I didn't anyways its better to teach him/her now than later because at this age everything sticks into his/her head oinstantly as they learn the fasinations of the world but don't forget to still talk to him in the one language that he/ she already knows