It is always confusing to decide whether to take the SAT or the ACT. I have a suggestion! You can have your children take a sample SAT test *and* ACT test, to see which one they score best in, instead of just guessing. Statistically, one third of students do better on one test, and another third do better on the other. But it is not always the test you think is the best! So try both tests at home first, before you decide.
You can get sample tests online. You can also go to the library and check out an SAT and ACT book, and take the sample tests. The librarian may even make copies for you of the tests. Each test is about 3 hours long, and I know it is a total pain to do it, but it is so worth it!
I just heard from a friend that her daughter got a full scholarship plus room and board, plus books and expenses, plus a $600 stipend to attend her first choice college! The mom said she was thankful for my help, so I asked her what was the most helpful thing I did. She credited my advice on sampling both tests. She said she would have never guessed that her daughter would do better on the ACT, and it totally did not make sense to her. But she tried my advice and pre-tested her daughter, and had her take the ACT instead. Not to over-state things, but it saved her gazillions of dollars for college!
Even if your children do not test well, they will likely do worse on one of the tests, so practice tests can still save you money. You do not know unless you try. If your children balk, you can give them a no-school day as a reward for taking the test, or bribe them with money. Whatever it takes!