When it comes to lighting baby portraits, natural light is the best choice. Flash will just end up spooking them. Try and position them near to a window and use a reflector to bounce light into any shadows.
The more light you can get onto your subject, the lower ISO sensitivity you can use for the best quality photos.
To catch a baby at their best, photograph them just after a feed or when they've woken up first thing in the morning.
They'll be more active and alert than at other times of the day, and you're more likely to get the kind of cooing baby portraits that parents will love.