Having a "common" Set means having things compatible to the base game so that you just can optionally add the rules.
Like the original extensions: You don't have to add Mercenaries. But because they are written compatible to the original game, you can add them whenever you like. Even in the Base Game Quests.
Important for that is, that the strength level of the newly invented things don't require anybody to re-write all the original quests. As soon as you have to touch the quests in order to keep them level with the Heroes, it is still a possible mod and absolutely ok for somebody's personal HQ version.
But then it is not compatible with everything anymore. That would mean it cannot be used in a "common" set to choose from (without rewriting the Quests i mean).
So for compatibility, what about writing it like this:
Courage (US + MadMickyG Mod):
Range: Ranged Magic
Cost: Cast action, discard card.
Effect: The next time the target attacks, he may roll two extra combat dice.
Optional: Discard additional fire spell cards and include one additional target per card discarded.
2 targets for 2 cards, 3 targets for 3 cards, and in case of custom spell sets (like the one of Slev) with more than 3 fire spells, you can add even more targets.
Advantages: No need to touch the game rules. You have an optional card you can use as exchange for the original spell. The card provides just more targets and - important for balance - has linear casting costs. The effect is somewhat better than linear because of course everybody will use it in the face of a big enemy group, but ok.
Because this way of modding it doesn't change the overall balance too much, you don't have to reinvent all the quests just to include it.
Change of perspective: Giving it to a Morcar Character would also be feasible because the heroes would do door blocking as defense and survive. If you can use it even in reversed perspective without killing entire groups, it is not too strong.
For your personal version, with your kids, you can make it a lot stronger - As it is just to teach them working together. They might want a big chunk of chocolate as payment as well.

Really, the pedagogic effect plus the fact that it is your personal version - i would do whatever is needed to reach the goal. So we maybe end up having two versions. One compatible and one for your kids.
