Personally, I believe a flexible work schedule is a privilege. I think it's a great thing and I think that it's totally fine to work non-standard hours, but I also think that if you are going to make the choice to not be in the office during core hours (I'd consider 9-5 "core" but I think companies should define this) then it's also your responsibility to be available during those core hours when it's necessary. If a meeting needs to be held at 3 (which is a completely reasonable meeting time within core hours) and you need to be at that meeting then I think it's on you to make accommodations for that by having someone else pick up your kids that day or making some other arrangements since it's your choice to work a nonstandard schedule. Same goes for a 9am meeting and someone who comes in at 11am. In a work environment where employees can make their own schedule I think having "core hours" where every employee is expected to be available at least sometimes is totally reasonable since otherwise your personal schedule is going to be negatively affecting your job performance, and that's a personal responsibility issue, not a job issue.