So, both hating on parents and doing acts of rebellion are part of teenagerhood. I
think we need to manage and not directly tell nor heavily imply "DON'T CONTRACT", because that might create the perfect moment for KB to swoop in while Minako is brooding and angsting and feeling rebellious and offer a contract.
Minako needs to make their own choice if the time comes, so they're gonna need information.
Of course, before getting to this... Minako might take literally any excuse to break their childhood image of Sayaka and Kyouko as the perfect parents, so... better they have a reason than not. I'd say, just admit to being scared.
Most importantly on the non magical side of things, is for Sayaka and Kyouko to continue to being supportive (but not overbearing) of Minako even if Minako doesn't want them to. So, sucks for Sayaka and Kyouko.
For the time being, I'm thinking...
[X] Admit to being scared to the point you couldn't bear to think about
this.
[X] Explain as far as Minako cares to listen.
-[X] Explain the dangers of Magical Girlhood. How lucky you were to made it past 14.
--[Q] And your overwhelming hate and fear of dice.
-[X] Use what Minako's seen: The girl just now, and the giant Grief Seed.
Now, even after everything, the Witchbomb is still a bitch. This would be a terrible moment to stop and tell Minako there's a further secret you can only reveal if they promis to not share it because of how dangerous it is blah blah blah... and omitting it also risks Minako not understanding everything well enough. So... sucks.