First and foremost it will be a great way to spend some quality time with your son or daughter. (Yes boys can work with yarn too!)
The charities have a lot of information on their websites, below is some general advice to help you get those needles clicking.
Whether you are nutty about knitting or an eager novice with your needles paused there are lots of charities to choose from. Knitting for charity can definitely be fun, but it can also be one of the most fulfilling and rewarding things you’ll ever do.
Yarn: Premature babies have very delicate skin and can often be allergic to wool; therefore it is best to steer clear of garments made with a wool mix. Acrylic yarn is more appropriate when knitting for preemies. Ideal yarn to use for preemie knits is baby double knit or 4ply yarn. Most preemie patterns will use these.
If you click on the resource link at the bottom of this article, you find hundreds of free patterns that will pique your interest. Under the image, Knitntyme specifies ”gauge/tension, number of stitches per 10 cm/4” or crochet hook. Suggested yarn may be replaced by yarn with a similar knitting tension.” So you don’t have to buy their yarn to download the pattern.