Beginner with Nikon D40

[Designer]

A portable flash with a swivel head is an excellent plan. The flash is very short-duration, which "freezes" motion, so shots you grab of a small child will be sharper than if you tried to grab that shot using available light only. There are some tricks that you will have to learn and use to avoid the straight-on over-exposed flash-in-the-face look that many people object to. To learn flash is easy and convenient, using online sources such as this one for example:

Strobist: Lighting 101: Introduction