A parent must know what level of math and any other subject, for that matter, that their child is performing at every school year. When children enter kindergarten and the school recommends that children know their ABC's and know how to count to twenty or whatever number, it is the parent's responsibility to make sure the child is prepared prior to enrolling in school.
Use things around the house that they already know and love! : My son has a HUGE adoration for his Thomas Trains... he can tell you every single trains name, number and color. When we started working on recognizing numbers and colors, I tried to use posters, games, workbooks, etc that would help him learn to recognize them... all to no avail! One day, we were playing with his trains, and HE asked me what number one of the trains was. Bells went off in my head and I realized that I could cater his natural curiosity about his beloved trains, and still be 'teaching' him to recognize numbers at the same time!
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.
Parents who are positively involved with their children and want the best for them will make sure their children are keeping up with their math skills and are successful in school.