K5learning is a great place to start. When I notice one of the kids is having an issue with a math idea I tend to do and find a worksheet related to it and kneel down next to them and help them through it. Then we go back to khan academy until I notice another road block. K5 does have paid recourses but I haven’t used them as of yet. I would like to buy some workbooks at some point and use those but for the moment printing our worksheets seems be be going okay. Best of luck with your boys, try not to feel overwhelmed at the start. :) in the long run they’ll learn what they need to know and you’ll notice what gaps need filled.