There was one book that I used with students who had trouble memorizing the multiplication facts. Basically, the started off by teaching students to double numbers, for example you'd say, "Double 3", the child would answer "6". this would be the two times tables. Then you would continue with this same progression and say, "Add 3 more", or child would say "9". They are learning to count in groups rather than memorizing just numbers. You can then practice counting by 3s (0, 3, 6, 9, 12...) I always had my students start with zero and then count up. You can even have fun with it and say something like, "You have until the count of 30 to get in the car and I'm counting by 3s." The more they hear and say it, the faster they will memorize them.
For some of my students they just didn't understand, but once they practice counting by 3, 4, 5, whatever, they got it. The book I am referring to is call Mental Math for the Middle grades by Jack Hope.
Whatever you do, keep after your children to learn them. It will make their lives much easier in the future.