You should probably mention the different school part, because from what my son said the tester brought that up.

What i did was I had told him that we wanted him to take the test and then we would all decide together what to do if he got in. Told him to try his best and we wouldn't force him to go if he got in and it wasn't a good fit. FWIW, my son enjoyed the test.

At my child's school they told the kids about it being to qualify for going to a different school. We mistakenly forgot to talk to our child beforehand.

we did tell our child about the test. And the prospect of getting into the "Center". (He was already at the host school and interested in the center.) But we didn't put pressure on him, and said there would be pros and cons to going, and that it isn't the best thing for everyone (including him, potentially). But that it's always good to do ones best on a test and to have options available.

We didn't make a big deal of the test. The night before, we just said that there was a test tomorrow at school. Kids are used to taking tests (map-m, map-r, reading tests, etc.), so it's not that unusual for a group to leave for testing.
My DC is at the center and we are underwhelmed. I wouldn't talk up any part of the process (or even the new school) to your child. At the end of the day, its just another classroom.

I'm pretty sure he'll learn about it from the other children so I'd recommend talking to him first. I told me son about all the opportunities at the "new school" but that it was very competitive. I played it out very casually. I also told him that our plan was to use the test results to advocate for harder work at this home school in the event he didn't get in so I talk about it as a "win win" for taking it.
My son did up at an HGC and it's been his best year yet. Great choice for our child.

My son is now in middle school, but at the time I told him this was a chance to go to another school that had a lot more neat projects and opportunities. I told him that there is a chance he wont be accepted and that if he is accepted we would include him in the decision. I said I want him to take the test to have a chance at these opportunities, but it doesn't mean we will do these opportunities. I also emphasized that not every kid does this and he is blessed to be in this situation.

In the end, he got into an HGC, but after looking into it, he really did not want to leave his friends, so we did not go. Being up front at the start, however, started a long discussion about the pros and cons. He really considered it carefully. Just going through this process was a good experience. I think we made the right call for him in the end.

I think your child will know from other kids exactly what the test is about even if you don't tell them. All there needs to be is one kid on the playground/bus/lunch table whose parents gave him/her all the information. My child had a classmates who parents were very determined she would attend (they were very unhappy at the neighborhood school). Though we were much more like take the test see what happens with our child, he got lots more input from her classmates. Luckily they both got in. My older child who did not take the test also new exactly what it was and which of his friends took it.

I did not tell DS about the program being in another school etc.

I did say that there is a program which allows kids to do lost of cool projects and fun things, but it is a small program and has only place for a few kids. So, the kids have to show that they can answer easy math and English questions and some puzzle type of questions and actually be able to sit for a LONG test.

I told him that not every kid will take the exam but his teacher thinks that he is one of the kids who can do this program. If he does not get in no big deal.

The test is actually quite easy. The trick is that you need to build the stamina to sit for such a long time and actually read the directions properly. Also, it is on the Scantron so they need to know how to mark the correct answer and not be one off.

Best of luck.

I signed up DC for the HGC test, but not everyone in the class will take it and I'm not sure how to explain to DC why he will be taking it but not the whole class will be. DC is prone to anxiety so I am trying to be as low key about this as I can! For the same reason I don't want to mention to DC that it is possible he could go to a different school next year related to this test. Thanks for any advice.