I have been told by my father in the trade industry that giving the client a deadline for payment and a call back to check in 15 minutes after said deadline is a good way to go. In extreme circumstances sometimes halting production till payment is given is also a good way?

Will do. Same for you also!

No more so get into the business of being an illustrator for authors or a publishing company. I have done art for a published children's book but the publisher had to trace over my art for copyright purposes. So I am looking for insight on what I should do to become more freelance or actually work