xtramath.org is a website and we also just downloaded mathemagica which is an app, if you're looking for drill and if your kids get excited about screen time (mine does).
I feel that practicing addition facts helped my son a lot, but we took a break and didn't "drill" subtraction, as he understood the process and was able to use addition facts to figure everything out fairly quickly. We haven't start multiplication facts yet, but we will probably do drill because, well, isn't that how everyone learns the times tables? ;)
ETA that one thing I like about xtramath is that it drills the way the previous poster suggests- starts with 0+, 1+, goes into doubles, and kind of works its way outward. (You've got 5+5 down pat? Good, then you won't have any trouble with 4+5 or 5+6, and once those are solid, it's on to 6+7 and 3+5).