This site provides great, fun math challenges for children of all ages. You can even sign up to have a math problem sent to you daily through email or on your phone or tablet.
Greg Tang Math has a wealth of really interesting, motivating math games that will challenge anyone . . . even parents! Kakooma is an especially fun way to practice computation skills at all levels of ability, but there are many other good games on the site. This site does carry advertising.
This will link to a PBS page that lists several math-related games. You will also find information about math activities to do away from the computer.
Investigations is the core math program we use to teach mathematics at RES. This link to the Investigations website lists several sites the publisher recommends for children in first and second grade. On the same page there is a link to activities that you can do offline with your child.
All the Trailblazers have been assigned accounts on Sumdog that allow them to practice math skills at their own level. As students gain fluency and accuracy in computation and problem solving, the program will give them progressively more difficult problems. If they are struggling with a skill, the game will give them more practice in that area.