Not what I meant. The Realm is a place of magic and magic, especially that kind, has a price. The children want to create a gateway between the Realm and Earth? No problem, they've just got to earn it. Thing is, they do, repeatedly, but then something happens and they have to do it all again. I truely believe Dungeon Master would just send them home if he could, he could easily keep them there if he wanted to but gives them many opportunities to get out. Hell, if it wasn;t for Shadow Demon they would have gotten home several times.
And yes, Venger is his son. And yet he never once sends the kids to try and redeem him, or even protect him. When they ask, he even gives them the knowledge they need to find out how to destroy him. Dungeon Master always gave the children a choice. He didn't force them to help others, he didn't force them to take on quests, they even ignore one of them completely. I can't even remember an instance of him lieing to them. He even actively comes to their defense on occasion.
Honestly, I'm pretty sure Dungeon Master's actual ability to interfere is fairly limited, which is why he keeps taking on pupils.