Look at it this way. (I will preface this by saying I don't have kids.)
What do you think would have happened if your parents had tried to explain sex, drugs, rape, murder, the Holocaust, world-ending calamities beyond your control, hellfire and total damnation, all at the age of 7?
I imagine some would say "Well I'd have assimilated that knowledge and been better for it."
I think for a lot of people though, they'd be traumatized/tempted. I know I went through D.A.R.E class and that lasted oh about 4 years.
Now make that "slavering eternities of hatred that thirst for your suffering and blood and are as much a part of your existence as the air you breathe."
I'm not saying continuing to keep the truth from his sons as long as he did was even remotely correct. What parent fails to check in on all their kids when one's doing coke, one's cutting himself and everyone else, another is already a certifiable homicidal maniac and a handful are just total dicks?
But you can read a parenting allegory into the desire to protect your kids from the awful truth. I guess I liken it to my dad a little bit. He had guns around the house for years, just bb guns and a 22 rifle. But he'd never told me about them really ever. I had to go find them, play with them, shoot them and misuse them a little bit. That's what it took. Should he have taught me about gun (warp) responsibility earlier? Sure. But there's no guarantee I would have been responsible with those guns (warp) even after he educated me. Magnus sure fucking wasn't, when it came down to it.
So cut the Emperor some slack. He's a single-working dad with about 4,000 full-time jobs and literally the weight of the universe on his shoulders. And then his pseudo ex-wife swoops in and steals the kids and turns them over to 20 different in-laws.
I suppose you could use Sex Ed. as a decent counter example. "Look Magnus, I know you're going to be off playing with the Warp behind the Coolant shed whether I'm around or not. The least you can do is do it safely."