You don't change how they are estimating, you change how you react to their estimates. This is the entire reason scrum estimates with story points, not hours/da

You don't change how they are estimating, you change how you react to their estimates. This is the entire reason scrum estimates with story points, not hours/days -- so that you let the team estimate however they want, observe how many points actually get done for a few sprints, then load future sprints to that level. And yes, if devs are off for a few days, do some arithmetic to shrink the expected velocity by whatever ratio of time you have missing people.