I'm a great fan of Whizz.com. It is really good for teaching children how to do the whole range of the maths curriculum. If you get an Answer wrong, it goes through the question again in stages and explains how to work out the answer.
The tutorial goes through a process and oh have to do,the exercises to move on, whereas many children will bypass questions they can't do or understand.
There are many many maths sites, but a lot of them are like filling in paper and can be very boring. Also, if they get the answer wrong, there is no explanation as to,why, so the child doesn't learn anything.
Whizz is more expensive. It cheaper than a real tutor but I believe a parent should be with the child to offer additional support.
We spend a lot of time trialling maths sites and this has been the best for filling in all the gaps not taught At school.
Mathletics is good for revision, but you need to,understand the topic. The tutorials can be very. Confusing especially if you don't really understand the topic.
There are lots of good video sites, but you need to know what suits your child and what makes them tick.
There are lots of fee paying cheaper alternatives but they don't cover,the whole. Curriculum.
I choose paying sites to get the records so I can monitor strengths and weaknesses and work with the teachers on areas I have found issues.
Off you want more,info am happy to help.but also search Whizz on Mumsnet because I have written lots about it in the past. And no I am not linked to,the company, just a mum who wants my kids to understand maths