We have all the resources of an Archmage at our disposal, including time travel for us but not for them. Training takes place in a fantasy world. 5 kids.
I like this prompt, and I think I see where you were thinking with it- or maybe it's just obvious to me...
We'd use our DF skills to dig a proper "dungeon" of course, full of various shinies made of rose gold and electrum (and trifle pewter and black bronze and other zany alloys). Then we'd summon monsters for them to fight and level up against!
Now I say "summon" because I like DND, and I like monsters, and in DND summoned creatures don't actually die! So there's no need for an actual blood bath- oh no our party died X_X
...Which is why we have time travel! We scoot back a few minutes, desummon one of the "baddies", and let them try again.
It'd be a combat-GM's dream: a series of encounters that the party JUUUUUUUST barely manages to win, every time! In reality (and many game systems) such a gauntlet would be deeply traumatic, but in DND everyone bounces back and rapidly levels up.
...I guess the question is, would that prepare them for life outside our training? Would they be reliant on "luck" to save them from any situation? I don't think so. If we allow them to escape encounters which are too hard (without time shenanigans) I think they would credit caution for their survival. They wouldn't ever know about their charmed timeline or any of the doomed ones, of course. Maybe we could even allow a couple of deaths, particularly if they "happened" to find some scrolls of Raise Dead a few levels prior.
There, easy! Time to release these level 20 psychopaths on the outside world, and try to remember why in the Hells we did this thing. Maybe to gain an epic level? Emulate a divine tier? Boredom? We're an archmage!