They can start with asking simple math questions and going on to harder questions, as the child gets older. Parents should continually ask children mathematical computation questions and give positive feedback when they respond. Children love to receive positive feedback and that encourages continues growth.
There was a time that the United States of America had some of the highest mathematic scores in the world for students in grades k through 12. A lot has changes since those days, and in order to help children get back on the path to being good mathematical students parents need to play an important role.
Go on a counting scavenger hunt! For young children, you can keep the hunt simple and indoors, go on a hunt and count how many bedrooms, windows, doors, bathrooms, etc are in your house. For older children, you can even take this adventure outside. Look for certain kinds of animals, plants, flowers, you name it! Add in another element of fun (for any age) and create a graph for them to track their finds!
Sort it out! The sky is the limit when it comes to finding things to sort! Candy, beans, cotton balls, paper clips... you name it! You can even change around how you want it sorted... one time, sort by color, the next time sort by size or shape! My son had a BLAST when I pulled his Matchbox cars to the school table and we sorted them by color! For younger children, you can make it easier by having 'color cards' and have them match the specific objects to the correct card. Older children can just be told to sort out the items by color, shape, etc.