I love using games to teach concepts, and this die game is an extremely versatile way to teach negative numbers. Ask your students to multiply, add or subtract the numbers they roll. This is also great for partner play.
This section of the BBC's site is designed for children learning to grasp elementary math. This specific page houses an online negative numbers quiz, and is great to assign as an additional resource for parents to take home.
This fun online math game allows your students to control spaceships rocketing around meteors. The only way to turn your ship is to answer certain questions right. All the questions are concerned with adding or subtracting negative and positive numbers, the next game in the series asks students to multiply and divide.
This book, by Stuart J. Murphy, is meant for third graders and his entire series deals with math. The following books in the series include: Divide and Ride, Tally O'Malley, and More or Less. I love how simple and interactive these books make math. They also kill two birds with one stone, since we can have our community reading hour together and students are asked to both focus on their reading skills while gaining some valuable insights into their math curriculum.
The 36 worksheets from Dad's Worksheets are helpful as short homework assignments. I love this site, not only because it's free, but because each worksheet given by Dad's Worksheets varies in ease, so you can better understand your student's knowledge of the material.
This page on the Common Core website discusses the standards for students are to reach by grade six. This includes being able to work with negative numbers with ease, and utilize them to describe real world phenomena: temperature, acceleration, etc.
This video series by Khan Academy goes over what negative numbers are, and how to multiply, divide, add and subtract them. This is a great series to give to students who are struggling with their homework, or to give out to parents in your newsletter because it's free. A lot of my student's parents love the free sites I offer them, and I always attach a short list of community and online resources to my monthly newsletters.