As far as I know, the parents being shiny has nothing to do with the egg hatching shiny. You could have both parents be shiny and the chances would be the same. There is one item and a method that does increase the chances. By completing the alola pokedex you get the shiny charm that ups the chances of a Pokemon being shiny (hatched or encountered). However, unlike past games, completed in sun and moon means that you have caught all Pokemon. The masuda method however can be done whenever. You just need to bread the Pokemon with a foreign counterpart. A good example of this would be someone in the U.S using a Japanese Pokemon or vice versa. Ditto works best for this.