Worksheets:
Math Drills has a variety of different worksheets to practice math skills and the answers are provided making it easy to check your child's work. Feel free to search for other topics to practice! Fraction and decimal worksheets are specifically helpful for 6th grade students.
Kuta Software has a variety of pre-algebra topics including free worsheets. Answers are also included on the worksheet samples.
Teaching:
Khan Academy has a variety of videos teaching a variety of concepts. It has more than just math! There is an app that can be downloaded for use on the iPad/iPod or Android based tablet/phone.
Practice Problems with Explanations:
XP Math - math games. You pick the topic and they have games for your child to play/practice math concepts.
IXL - You pick the grade level and the concept you want to practice. If the answer is incorrect, IXL shows the steps necessary to get the correct answer. The site is customized for the problems to get more difficult or easier depending on the response given. You are limited to 20 questions per day unless you pay for a subscription.
Adapted Mind - You pick the grade level and the concept you want to practice. If the answer is incorrect, Adapted Mind shows the steps necessary to get the correct answers. The site is customized for the problems to get more difficult or easier depending on the response given. You are limited to 20 questions per day unless you pay for a subscription.
Games:
Fun Brain - They have a math arcade. Your child can practice skills while playing a math game.