Hi there I have experience of teaching children and have Enhanced Disclosure. I would say that it would be best to start lessons at about 6/7/8 years old, just to make sure they are genuinely interested and their attention span isn't too bad. As a teacher I would have to adapt my approach for young children. You have to be careful not to strain the voice as you only have a small vocal range when you are young. Lessons would be half vocal technique and half learning about rhythm, reading music, improvisation, etc. cheers