Skoolbo http://skoolbo.com/ is a free website that is being developed by the people who created Mathletics / Spellodrome. It combines both spelling and numeracy, and is great fun to use. It's good for all platforms: iPad, PC/Mac etc.

My son's school offers the children a Mathletics account, but his teacher set him at a level which is way below his capabilities, and no amount of chasing around has helped him move up any level, despite his cries of boredom and frustration, so I have purchased a separate account for him (approx $50/year) This allows me full control over weekly 'tasks' or assignments and numeracy subjects...and I set the system to match his learning levels. It also allows for a variety of school systems - from International Baccalaureate to localised state curriculum.
www.mathletics.com

There's lots of Apps that are free too. Do a search under "education / maths" and see what sparks your child's interest and run with it.