I will add...
We sometime use Singapore Maths books for my DS8 (we started when he was 7), but not from the company "Singapore Math Inc.". We have the current maths books used in the public school system in Singapore. I hope that hasn't confused you your cause, but I wanted to go to the source! They weren't as easy to get as I had expected. I wonder how different they are at the moment?
We find the maths book with explanations and some problems are enough (we don't use the workbooks). The colourful pages are a big bonus.
He also finds the Beast Academy fun to read, again no workbooks. Unfortunate, it doesn't go further but your younger guys might like it.
