Beowulf is a great story which is based on fiction by revealing the protagonist "Beowulf" fighting an imaginary creatures and monsters, on the other hand; some readers think that the monsters in Beowulf turn an otherwise serious work into a story for children which in my opinion, it's not a story for children.
First of all, the monsters in Beowulf such as, Grendel, Grendel's mother and the dragon are all made of fiction and they symbolize fear. Children believe everything they read or watch. Also, children can't differentiate between reality and fiction, which exists in this story while adults, can feel its beauty.
Second, choosing the dragon to be the guard of the treasures cave was a nice idea although, it sounds little factionary. Children don't know what a dragon is or why did the author chose it to be the guard while adults do. As I said Dragons symbolizes fear for being scary, huge and throwing fire out of their mouths, that's why I think it was a perfect idea from the author for choosing a dragon as a guard.
Also, the violence in the story is not suitable for children. For example, when Beowulf fought Grendel he cut his arm and his head off, also the author described a lot of violence scenes such as, when he described the blood bubbling in the water and how the dragon got angry and attacked the people and wounded Beowulf.
Finally, the main reason for calling this story against children, is because every scene in it has a meaning and every monster symbolize a meaning too, which children can't get. They might enjoy it but can't feel its beauty.