Yeah, it really doesn't matter what they say, based on those images for the YouTube videos they are 100% marketing to kids. Cigarette companies made the same claim about who their audience is but they were still forced to remove cartoon characters and colourful packaging from their branding because it appeals to kids, regardless of their stated intent.
You can be an adult who enjoys kids things like Let's Plays (hell, I am an adult who enjoys kids things like videogames and Lego!) but that doesn't change the fact that they are for kids.
To advance this conversation in the most constructive manner, please watch all of the above videos at the time stamps given. It's not very long, but it will give you a very clear sense of where I am coming from.
Saying that this is for kids is like saying
Deadpool is for kids just because it's a superhero film. Parents can bring their kids to watch -- and many did -- but it doesn't mean it's for them. The creator of the show is an animator. Just because something is animated does not mean it is aimed at children. The entire Adult Swim block on Cartoon Network is an example of animation not intended for children, but even that is PG-13.
The language used on Game Grumps is stronger than what is found in
Deadpool. This is content you would not want your children to hear. In fact, most people aren't even really there for the gameplay but for the anecdotes, jokes, and commentary; it's effectively a visual podcast with comedic moments of gameplay-related ineptitude (and sometimes triumph) sprinkled in. The commentary comes before the gameplay, although the gameplay sometimes aids the commentary.
It's not for kids. That said, there's nothing
visually suggestive on the show (note the thumbnail on the left is censored where appropriate), so nothing on the channel is flagged as mature content.