물가에 내놓은 애 같다: You’re like a child by the water.
물가: a riverside, a shore, the waterside
내놓다: to release, set free
애: a child
→ Like a child released/set free by the water.
→ You don’t know what children are capable of doing in any situations. When they’re by the water, they can try to swim, to jump in, etc. They put themselves in danger without knowing. When you say someone is 물가에 내놓은 애 같다, it means you’re worried about them because you think of them like a child by the water, putting themselves in danger without knowing they are.