Cairn terrier claws are to sharp!?

Our cairn terrier is 9 months old and already we have had to take her for her claws trimmed 4 times. She gets regular walks (2 x 30 mins to school every day along the pavement and a trip to the park atleast every other day) yet her nails are razor sharp. she also has a bad habbit that when we are playing she will grab/pull ur leg or arm with her paws to get ur attention but this means she scraps her claws all down ur skin (causing a lot of upset with our 4 year old son). Does any one have any suggestions? Is there anything we can buy to perhaps file the edge of her nails down? i have heard you can clip them youself but this can be very dangerous if u dont no what ur doing (which i dont!) x