As well as Mathletics and TT Rockstars, I would recommend Mathsbot PRIMARY and opening a free account at Third Space Learning.

We use Mathsbot in class for the morning basic skills sessions and I use it to generate their weekly arithmetic homework tasks. You can access all year groups and there are practice SATs papers that autogenerate new questions. They also have video explanations of how to solve arithmetic questions.

Third Space has a wealth of questions that you can access for free. I'd recommend the rapid reasoning resources if you'd like to challenge your child or gain an understanding of how questions can be presented. 

Good luck, enjoy and let me know if you need any further assitance.

Mr. Line