While I agree Big Boss was more like a gray villain, the bolded part is not entirely correct.
Big Boss left The Boss ideals back in Peace Walker (he says as much in the ending). His ideal is to fuel war in an endless cycle so he and his soldiers can have a place in the world and don't become relics of the past. He sees a world walking towards peace and he doesn't want that. He doesn't take children so they can have a better future (that's Venom, and it's one of the many contrasts shown between him and his "demon", aka Big Boss), he takes children to more effectively perpetuate the cycle of war.
Here's part of his speech in MG2:
"You saw those children, didn't you? Every one is a victim of a war somewhere in the world. And they will make fine soldiers in the next. Start a war, fan it's flames, create victims... Then save them, train them... And feed them back onto the battlefield. It's a perfectly logical system."
That being said, people expected Big Boss to turn evil because of terrible marketing that made everybody forget past games. He basically turns into MG2 Big Boss in the end of Peace Walker, which is precisely when he tells Kaz "you can call me Big Boss now".