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.
Get in the Kitchen! I was very hesitant for a while with my son when it came to the kitchen. I worried about the GIANT mess that would be left when he was done "helping". You don't even have to take on the entire project with them! You can start small and let your child help with measuring certain ingredients. You can modify the difficulty of the 'lesson' by talking about the different kinds of measuring cups and seeing what other measurements can also add up to equal one cup, etc.
Also, when the child is in the second or third grade and their child can't add simple two digit numbers, it is up to the parent to make sure that their child gets the extra help needed to become successful. Parents cannot wait on the school every time their child has an academic deficiency. Part of being a good parent is making sure that your child functions at the correct grade level throughout their academic career.
Go Shopping! : You can cater this to all different ages. For young children, you can do a 'mock shop' and let them pretend to go shopping in a pretend store (your kitchen). Label certain items with easy to read prices and print some pretend money for them to shop with! For older children, you can let them help you in the actual grocery store with a specific shopping list. You can even set specific spending limits and give a specific amount of items and let them determine what they can afford, etc.