For maths, subscription but pricy - Whizz.com. Lessons are based on ability rather than age/year. Excellent teaching methods.
Cheaper version for maths - Mathletics. Great for practice and consolidation, not as good on teaching.
EducationCity. Cheap, good basic skills in Maths, Literacy, Science, French German and Spanish. Language sections are quite hard to understand so not sure how effective it is. Again good for covering the basics though.
Lots and lots of free maths. Type KS2 maths and hundreds of sites will come up. SOmetimes variety means they can practice the same topic in different ways - helps their understanding.
Both Mathletics and Whizz follow a formula that kids soon pick up, but then they cant always translate it into different maths scenarios
Upsite of subscription sites is you can track their progress. Downside is there are lots of free sites that are just as good or even better with their teaching and ability to practice a huge variety of topics