With 600 words, you're looking at flash fiction. I've done a fair bit of that, but it requires extreme focus. Doing both the fight and the journey won't work very well.
Pick either the fight and immediate aftermath or the journey and go with that. Have the first part show of the state of the girl and her baby brother, fear, hunger, despair, depression, hope, determination, etc, and really focus on that.
Then as she's picking through the ruined supplies for anything salvageable, or as she's trembling from exhaustion and hunger after walking all night, she hears the twig snap behind her and smells cologne, or blood, or sweat in the breeze. She looks behind her and...
At this point you could leave it ambiguous, letting the reader decide if it's someone who can help her or an enemy. Or you could have her giving a tentative smile at the newcomer. Or she falls to her knees crying and holding her brother to her chest, staring at the ground as a gunshot rings out.
Short and sweet is almost always the way to go with flash fiction. Let the reader fill in most of the details.