Having a Y4/5/6 class is a great challenge as a teacher as I am constantly looking for open tasks that challenge all my students wherever they are.
Have been having a bit of a focus on maths lately and a few sites have been helpful but generally there is a HUGE lack of rich math tasks online. I found this really surprising! I am used to finding whatever I need/want online.
Anyway.... check these out.
Am also adding
https://www.khanacademy.org - lots of how to videos that poppets understand better than me. Account easy to make, especially if you want to sign in with gmail or Facebook.
(I haven't used dragon box myself yet but highly recommended by some alternative educators I know. Will be setting up one of my year 6 boys with an account as he wants to learn algebra! The other year 6 boys is going to do Khan Academy.)
Advise all teachers to have a Khan account too! It's good for you!