The boys fell down and landed on their buttocks. Fortunately, the branch they were on was not too high up, otherwise they would have been seriously injured.
Is the word 'up' necessary? Are there any mistakes in the sentence?
Do you think as Marius does? He says that I should write "The branch was not high up in the tree." By adding 'in the tree' would it make the sentence better? Or is it understood that the branch is in the tree? In other words, "in the tree" need not be added.