In order to guarantee an IV gets passed down from parents to children, you must use either a Power item, or a Destiny Knot. Without these items, there is no chance of passing down IVs intentionally. They might still roll 31s naturally, but that is down to RNG.
The Power items will guarantee the parent holding it passes down the IV corresponding to the Power item. If either parent is holding the Destiny Knot, 5 IVs will be chosen from among the 12 available between the 2 parents, and pass those down. I believe if both parents have the same IV chosen, the higher one will be taken.
It is possible to get a Pokemon to have 6 IVs without hacking. I have just done so with a Froakie yesterday, in fact. But it is ultimately up to RNG and persistence to achieve it.