Technically in English, we sometimes use the phrase "We are no..." but it's rare. An exception would be "We're no angels" or "I'm no angel" which means "we aren't perfect" or "I'm not perfect".
Another thing we use it for is for profession. "I'm no scientist, but I think the Earth is round". or "I'm no mechanic but the car has a problem"
You were not that far off, but in this instance we would use "not" because it negates the noun and tells us whether or not someone is or isn't a child.