If I had control over regencies, I would give them as much control over the realm as possible, especially the council. If the regent wants to give all the council jobs to his friends and followers, that's within his power. If the regent wants to go on campaign, he calls his banners and away they go. If the regent wants to plunder the royal treasury to line his own pockets, who's going to stop him? The counterweight to all of this being: whatever negative opinion modifiers the king would usually acquire from this behavior are inflicted upon the regent, and magnified. The regent is going to get considerably more pushback from the nobility if he tries to exploit or cripple them, and there will undoubtedly be others seeking to advance their own candidacy for the regency. If you can obtain the regency, it should be a path to great power, and a means to enrich your own house, king in all but name, but there will be consequences for that behavior, especially once the child-king comes of age. On the other hand, a good, loyal, and skilled regent may find himself gainfully employed and gifted with royal honors for many years to come.