I wouldn't worry too much about it. Here are my thoughts:
1. The WPPSI is sometimes described as not the best choice for HG+ children (although most often those statements are made with children closer to 6-7 years old).
2. IQ testing with younger children is a bit less accurate than with older ones.
3. I think that the tendency of children from the same family to have similar IQ scores has possibly been a bit overpromoted. I'm sure that children do cluster together that way, but it's not so uncommon for children to score more than 10 points apart either.
4. You're dealing with a lot of sources of variability: underlying variance in g; differences in test-taking skills; variability due to younger age (which may be the result of many factors including different academic exposure, even in the same household); different rapport with the tester; and differences in physical and emotional condition on test day. Kids are just different, and testing young children is less accurate than normal.
5. Even if rushing on the part of the tester may have been a factor, there's not much you can do about it now, except test in the future with a different person.
6. Your son's score likely meets whatever cutoff is used for your daughter's school. If so, what does it matter?
