That's a really good point.

Test preparation. Any child who is coached on general knowledge/content/format/scoring principles will obtain an invalid score.

Any score on a standardised test is only valid if the test-taker, the test administrator, and the protocol are the same as that used in the standardisation sample.

Preparing children for IQ tests = invalid index scores.

You may as well send yourself a letter saying, "Your child is gifted" as trust in the index score of a child who was prepared for the test.