This is obviously a simplification to remove one layer of tactics and management for my kids. It would be nice later on, when they are more experienced with the game, to restore the full rules on this.
How are you normally handling this aspect of the game, do you always enforce the rules strictly or do you handwave something?
I go to for a strict ruling. I want to know light sources and ranges. If there is, for example, weapon fire from the darkness then I tell the players, "You can't see where its coming from." I also enforce the disadvantage on WIS\Perception for characters with darkvisison (which so many tables ignore, either accidentally or deliberately).
It is important to remember that being in the darkness makes you unseen (AKA concealed) but does not necessarily make you hidden. Unseen and hidden are two different things - something many people don't understand (mainly because the rulebooks explain it really badly

.
Unseen means someone can't see you but knows where you are. For example, you are in darkness or you are invisible (but leaving tracks and/or making noise). They can attack you, but at disadvantage.
Hidden means someone does not know where you are, or even that you are there at all. They may know you are there, but not know your location with any accuracy (The thief is somewhere in that half of the warehouse!"). If they try to attack you they have to first guess at your location, and even then they are attacking at disadvantage.
Just being unseen usually does not make you hidden - you have to actually take some actions to conceal your presence - but being unseen is usually a prerequisite for attempting to hide. For example, you are in darkness or invisible and have taken steps to conceal your presence (by muffling your breathing, walking slowly and carefully, silencing noisy equipment - which is usually modelled in the game by making a DEX\Stealth roll).
Note that it is also possible to be hidden but not unseen, though this is rare. This is usually known the terms "disguised" and "camouflaged".