This is a question I get asked a lot – especially by parents who are holding new-born babies in their arms. My recommendation would be to start getting them accustomed to the sound of the language you have chosen straight away – not by engaging a home tutor, but by playing them a CD of nursery rhymes at least once a day.
Wait until they are at least 3-5 before you think about having a language tutor, and then consider making it a family learning experience rather than a lesson just for your child. This will make the experience less intimidating for your child, will enable you all to practise together in between visits from your tutor, and will help your child retain the language better.