Well, they would be busy due to the online homework. However, if you want to keep them busy with additional work, then I would recommend the following.

Twinkl:

It has some free resources. The issue with Twinkl is that you need to take the print out of their resources, as they have nothing online tasks. So, there would be some additional costs you need to keep in mind. One of the drawbacks is that they have tonnes of information, and you need to find the relevant materials. I think they need to compile their information in a better way. Their website is quite overwhelming.

Ixl:

It is also a paid platform, but the good thing is that everything is available online. There are hundreds of assessments for both English and maths. It has a better interface. Kids can quickly find out what is relevant for them and easy to see the progress.