Welcome and congratulations on the baby!
The built-in flash is enough for portraits.
However, some people prefer not to use a flash for portraits at all, due to the feeling of "unnatural" color it leaves and the "unnatural" shadows it casts. A good external flash will allow you to point its light upwards, thus reducing these effects and you can use a plastic piece (not sure how it's called) to diffuse the light.
But you could try using a higher ISO setting, maybe ISO 800 or ISO 1600 or something, and see if that gets you the desired effect, without the need for a flash.
Myself, with a Nikon D40, have taken portraits indoors at those ISO settings and got a sharp photos, with natural colors. And there's always the trick of placing your subject next to a window (with transparent curtains).
DSLRs are much better than compacts at low light photography. Take advantage of that. Experiment with an adult subject first and see what works for you.