First Grade Maths may just be the beginning, but it's the most crucial stage as this is the foundation of your children's future. If they take a liking to Maths today, then they'd do really well even in the higher classes - because, interest is the spark to all learning processes.
Kids aren't under any stress at this age and therefore, they could easily grasp knowledge through fan and game. Anything that's enjoyed is easy to remember and hence they'll grasp the concepts easily.
This article has a list of various things that you could do to make this learning curve more of an adventure rather than a chore. Take a look at some of the best workbooks, online games, Android games as well as free math games that you could play with your kids.