But they don't try. Like her father is incredibly creepy, but we don't know if that is on his own, or powered by Pennywise. We never really get that answer, we never get to see them even try and get others to see it. They just assume adults won't care, won't see it, won't believe. We don't even know if Pennywise can target individuals who see what he is showing, without him showing it to others.
Also, adults can see the blood, they can see Pennywise. Its just harder. Otherwise how is part 2 going to happen?
Except that we do get that answer:
Bill sees Georgie. Bills parents dont want to hear another word about Georgie. Georgie is dead and theres nothing left to find. They want him to stop trying to look for him. Even when the losers split up after the incident at the house, there's a shot of Bill sitting all alone at the dinner table. His parents aren't spending time with him anymore. Can he really tell them he saw Georgie in basement? Hes not there now.
Mike sees burning hands reaching through a door, which opens to reveal Pennywise inside. An adult comes through that door a minute later, having not seen any of it. Hows he supposed to believe what Mike just saw? Theres nothing there.
Ben sees smoldering Easter Eggs in the library, gets chased by a headless boy, then by Pennywise, and straight into a librarian, who didnt see a thing. Pennywise chased him right into an adult who didnt see him. Hows Ben supposed to explain what just happened again?
Stan saw the lady disappear from the painting that he was already scared of, who chased him through his fathers synagogue. Was someone supposed to believe that happened?
Eddie was chased by a leper on a street with a bunch of abandoned houses (one that was referred to as a crackhouse later in the movie). We saw his mother. Exactly how was his mother going to deal with that if he told her. He couldnt even let her find out they were buying first aid supplies for Ben.
Richie...he hadnt seen it. And after he did, he wanted to quit and pretend it didnt happen, not run to an adult. Getting an adult meant sending people back into the house, and he wasnt going back there.
Beverly had gallons of blood drench her bathroom. Her father didnt see it. He didnt miss it because he was creepy. He didnt see it because Pennywise kept it that way. He wanted to make Beverly have to live with another horrible secret. And that failed when she brought the rest of the losers in to clean it. They didnt know it when they did it, but it was the first time they outsmarted Pennywise.
Theres no grownups cant see it rule with the story. Its more about how grownups cant help you, and about being betrayed by the people who are supposed to love and protect you. Its not that they assume grownups wont care, its that they already know that the grownups dont care or wont help. Pennywise feeds on that.