I would expect both to feature next season in a small way. Brahim's already had his debut and Sancho is definitely ready for it. Pep said he's not brought the kids in this season because he can't risk bringing them into a rocking boat, he needs things to be more settled.
I assume that means we will see him fulfil that part of his job description next season, the attack is already finding a rhythm and we're controlling games more, so by next August we should have that stable platform for a kid or two to come into.
We can't expect other clubs to "take risks on our behalf" as you put it because they know, everyone in the loan system, knows there's no benefit to them. Why spend a year watching them cock up if they're walking away in 9 months time? We saw with Roberts even Celtic weren't prepared to loan him unless he was there for 18 months and they got some sort of dividend for the investment.
And that's not to mention the fact that they're our academy's crown jewels and we can't be loaning them out to some random club in the football league who fancies a punt only to see them set back significantly by bad tactics management and a spell on the bench.
We should be bringing them through, and if we do, there's every chance that we've got our own Mbappé calibre players in 18 months time.
If we're going after Mbappé anyway, his contract expires in 2019. We should be all over him, his agent and his family to try and either create a Lewandowski scenario in 2019 (when he'd be 20 still) or get him for a reasonable price in summer 2018 and recompense him 2x for the salary he's lost by not signing a big deal with Monaco.