See, it's not about whether or not their name blew up. It's more about whether they were right for the role or not, and if the writing matched and synchronized or not. I can tell you from experience alone from being in a casting director's position when I was in Hollywood that a lot of people in the same position as me liked to go "Oh, he's a new up and comer, he looks great! Let's get him!" without even looking as to whether the script is best for their talent or if they were right for the script. If Tenkuji Takeru is a boring character, then that makes the actor a boring actor to look at. If the actor is a boring actor, then no matter how good the script, the actor makes the script sucks. It's a symbiotic relationship that you NEED to get down perfectly, and that only comes from understanding the written material, and having an eye for pulling out the correct and right actors for the part.
And, we CAN deny it is a child's show. Just look at Kamen Rider's history as to what it started off as, and what most of it did back then. Sure, you can argue that people have become more sensitive, therefore things like Shin Kamen Rider and other more gorey and violent shows wouldn't exist, but the existence of Amazons alone disproves this - because it tried to take Kamen Rider back to it's gorey nature that it had to it as well. It's the same argument you're making for Kamen Rider that people who make against comic book fans if they complained the movie was subpar compared to the source material when the movies Marvel and DC makes gets watered and dumbed down. It's just false equivalency.
And no, my nephew wasn't the only one questioning it. I said his friends (3 of them) all did. So that's four kids. And I can say that all of aren't "too smart". See, this is what I mean. It's insulting the intelligence of the common people that if they can just see past bullshit, somehow they're smart. There's actually a study done that people's IQ levels, especially in kid's, increase when they are doing things they love and when they are happily engaging in activities they already liked, so even intelligence level varies depending on the situation and context of things.