Is it common for children to be really farsighted, even if their parents have normal vision?

It's normal: Most children are born about 6 diopers farsighted, 'cause their eyes are small (90% of adult size). This is completely normal. Most three year olds are about 3 diopters farsighted. As kids grow, their eyes get longer, and they reach "normal" sight around age 7-10. Kids who are a lot more farsighted than this may developed a crossed eye which should be treated to prevent amblyopia.