In Vilous: Soldier of the North, a parent map is an open world map that allows the player to explore the environment in it's totality. The only boundaries are the edges of the map, which have a soft kill zone.
Aside from important game characters walking around to find you when needed, a number of standard NPCs can also be observed roaming the world of Vilous. Until a certain threshold is reached, players will notice that the farther North you travel, the more Northern soldiers you will see wandering around on patrol. There is an inverse effect for the farther you travel South. While moving South, players will notice a larger number of Southern soldiers until a certain threshold is reached. There is not a significant number of patrols roaming around to make the game seem more realistic and to prevent it from turning into a bloodbath.
There is an average of one two man patrol per every 1.5km^2, per clan. The number of patrols a clan has will increase and decrease based on their location in the world and their relative position to their main city. Because there is such a small number of patrols per unit of area, they are not to be treated as a passive entity. Patrols of opposing sides will attack one another on sight and will also attack the player, should the patrol be from the Reono clan. Southern recon teams can be deadly if not handled properly. Players should always keep their ears open for idle chatter or footsteps.
Child maps are just limited replicas of the parent map. Each child map is specific to the story mission that it's related to.
Child maps are full maps of Vilous that have been limited to the area around the intended area of play. Eventually, should the player stray too far out of bounds, they will encounter a soft kill zone. Beyond that, if they somehow manage to make it deep enough into the soft kill zone, they'll simply fall into the void, as there is no map beyond the intended play area.
In relation to story missions, child maps have heavy scripting in all necessary areas. Child maps will also have independent time scales, weather, and climate that is based on the requirements for the mission to be executed as planned. Child maps also lack NPCs wandering around. Anything that happens on a child map is intended to happen and nothing is random.
To the right is a visual representation of how mapping works in V:SotN. This image was created by Ryan_kuke in order to better explain the concept to a devTeam member.