In the given sentence, the correct word to fill in the blank is 'they're,' which is a contraction for 'they are.' This contraction is used to indicate that the girls should be careful because they may fall. Using 'they're' instead of 'they are' makes the sentence more concise and fluid. It's important to understand the context of the sentence, which refers to multiple girls, and the need for caution due to the potential risk of falling.