I don't really have a good sense of what would be best for an 8-year old. I'm a little overly sensitive since mine is only 2. There are hand-to-hand fighting scenes, a large scale war, and both large violence (explosives killing multiple people) and individual character deaths.
I'd trust the reviews since they probably have a better bead on things. If it heads in the direction it looks like it's going to be a pretty deep show. Here's how one review put it:
In the original series, the ThunderCats’ foes were simply known as mutants (although they were reptilian in appearance) — in the updated show they’re called Lizards, a species that also inhabits Third Earth and has grown resentful of the Cats’ power and superiority. The series is certainly positioned to comment on larger themes of race, equality and globalization, and there are hints of those plot threads in the pilot, which will hopefully be explored further as the series progresses.
I'll add science vs. faith/mysticism to that list as well. If you feel comfortable discussing those themes week to week, I highly recommend it.
One more thing to add. The pilot ep may be a little overwhelming. I'm not kidding when I say it's epic in scale. It feels like a movie.