The EXPLORE might also be an option if you are mainly interested for DYS purposes. It would be a lot shorter, but it is offered less frequently.
While it defeats the purpose of talent search if students prep for exams, it would certainly be reasonable to get a book from the library and have your student take one sample ACT or SAT test at home. You could get a ballpark of how they might score and it would help them get used to the directions and to bubbling process.
Does it help or if the child doesn't do well, does it have a negative effect on their self-steem?
I would explain to any kid that it is different from other tests you have taken in your life. You can get A LOT of questions wrong and still do very well. As a younger student they should expect there is quite a lot on the exam they've never seen before and don't know. That's okay. If your kid tends to be bugged by this it is a good reason to take the ACT because it doesn't have a scoring penalty so they can guess on every question and lose nothing.
As far as "not doing well" - I'd say that's all in how you define the bar of success. If they can get anything right on a test designed for students getting ready to go to college they are doing great.