I have hunted out of multiple ground blinds, with multiple kids of various ages, with both crossbows and guns over the years. Skip the ground blind if possible, it will frequently hurt you more than it will help. Best advice I can give is put up a 2 man tree stand. Bring an iPad, trust me on this. 8 year olds have a short attention span, getting bored easily when they are not seeing much.
Biggest problem is there are too many blind spots when hunting out of a ground blind. With a kid you need as much advanced notice as possible when deer are approaching. Because of being in the blind and the blind spots, sometimes you don’t see or hear the deer until they are right in front of you. When that happens, it is typically too late with a young kid. Don’t forget you are at eye level with the deer. The deer will still see movement in the blind. Had multiple deer look right in the blind. Game over with an 8 year old.
Another thing, with a ground blind, sometimes you can hear the deer, but often can’t see them due to the blind spots. It becomes a guess as to where they will show themselves. You can hear them. The deer are close. You cannot see them or determine the exact path they are traveling. Tough to get the kid set up in the correct position. You may think the deer are going to show up to the left of the blind, and you look up with the deer being a few yards away on the right. Again, game over. Hard to reposition the kid under such circumstances. Especially with an excited 8 year old who cannot contain themself.
Two man tree stands offer a huge advantage. You typically get way more advanced notice which is critically important with an 8 year old in archery season. Much more time to correctly position your child for a shot. A little movement often isn’t fatal. You can whisper to you child, give instruction, without getting busted. And it is easier to see what your child is doing. If they are fumbling or something, it is much easier to get them straightened out without getting busted by the deer.
Personally, based on my experience, I would avoid hunting out of a ground blind with a young inexperienced hunter if possible. Young hunters will make every mistake possible. Mistakes are much harder to over come when you are at eye level with a deer 10 or 20 yards away.