Have your child take the test early— definitely by sophomore year. They will take it with no pressure, better understand what is on the test and be better prepared later.
The other test dates do not. This is extremely valuable as many students will say “I have never learned some of the math.” or “I didn't do well on science because it was all physics and I have not had physics.” The reality however is that a student does not need Physics or high-level math to do well on the test, and getting their test back and being able to review it with a professional will help them understand this. It will also help them better prepare in the future and recognize the strategies needed more effectively.
If you can afford it, send them later. You don't want to send a poor score to a school. It is better to achieve a solid score and then determine where you would like to send it.
(Unless required by the school - typically only a handful of Ivy League schools require all previous tests). Most schools want only your best score.
The composite score or ‘ACT score’ everyone discusses is actually comprised of the 4 subscores students receive on the English, Math, Reading and Science sections, added together and divided by 4. When superscoring, you can take the Math from one test and combine it with the Science from another and use still another test for Reading and English if needed.
Colleges determine if they superscore - DO NOT ASSUME the colleges to which you apply will superscore.