Sounds like you have a 2nd, a 5th, and a 6th grader. 

Go here:  

http://www.ixl.com/

and click on the grades your kids are in, then click on the 
various skills. There are lots of problems for every skill 
in every grade on that site.  You can do a lot on there for
free, but sooner or later your time is up unless you join,
but you might want to join at 10 bucks a month.

Hope this helps.

Edwin