I suggest you have a full on PARENT MEETING every May at end of year (or August at beginning) where you release the calendar. One where you get forms filled out, releases signed, papers done, and EVERY parent has to meet you and acknowledge everything with you IN PERSON. Make attendance 100% mandatory or the kid's schedule is changed and they're out of band. Make it be a two-day event for the parents who can't make the first day, if you need to. Get your coaches to grill some hot dogs or burgers, and make it a cookout - people come to food!
At that meeting, MAKE them sign up for Remind or your equivalent active messaging app. Make it a requirement. Then, using iCalendar or Google calendar, SEND OUT INVITATIONS to EVERY single event so that it gets PUT ON THEIR PHONE CALENDAR. Make it impossible to ignore. When you send Remind texts, REQUIRE that they respond or re-send it 12, 24, whatever interval hours later.
Even with ALL of this, you'll still have kids with parents who choose to go out of town. You won't be able to stop all of them - but if you show that you mean business, and you actually do mean business, you'll get change. Turning the rudder on this ship can take years.