Test scores can vary by the day or hour by 5 points. Whatever test you choose just read about the test administration of each subtest ahead and get a sense of whether there is anything in particular in the test process itself that would be particularly difficult for your particular child.
For example some sections are timed, if timed tests happen to make your child panic, you can work on that ahead by playing around timing non academic things for fun at home like how long dinner takes to eat.
A skill taught at some schools is to not beat around the bush answering or to keep answers short or even that the best answer is always "I don't know" because what the teacher really wants is for the child to be quiet so they can "teach". But IQ tests reward the opposite in answering. If a child reflexively answers "I don't know" to most questions, be sure to explain ahead that in these types of tests they like people to guess.
If he can't stand not knowing an answer immediately after answering, explain ahead the tester likely will not tell him the answers as the test progresses. This can be debilitating for some kids, hung up on knowing whether they were right or not or just plain too curious about the answer to progress mentally fully to the next question.
If they are a child who you have to remind to get around to actually answering a question or keep them on track, if you have trouble getting their attention sometimes, then working on focusing isn't a bad idea. In order to be consistent between children, the tester is limited in how much they can remind them of the rules or repeat questions, especially in the memory sections. If a child has a fantastic memory but lapses on paying attention at the wrong moment, there isn't much the tester can do in terms of repeating the question.
I'm not counseling prepping for an IQ test in terms of content, but more saying that the administration technique can favor kids with some experiences or personalities more than others. It's just worth a few minutes to read up to see if any of the administration details would be hardships for your particular child.
If there is any area in which your child has an actual disability, such as fine motor skills, then research ahead about subtest substitution to see if administration of a different subtest would be more accurate in their case. Don't expect the tester to know more about it than you.
