Sounds like your bosses, or the scheduler, don't care very much since they "allowed" the other part timer to go. I hope, assume, she told them why she was leaving.
I don't remember how early the schedules came out where I've worked.
I worked full time at a smallish out patient surgery clinic for 5 years. We rotated shifts, 4 weeks we were 6 - 2:30, 4 weeks you were 6:30 to 3, 4 weeks 8 to 4:30, 4 weeks 10 to 6:30, plus a few other various shifts in between the early and late ones. That was real nice to know when you were going to be the early nurse or the late nurse for 4 weeks at a time.
Then I went per diem to a large out patient surgery clinic. It took them, and me, a while to sort things out, and who did the scheduling changed several times. But the scheduler always knew who had child care needs, i.e., couldn't work certain hours, and was able to accommodate them. Every scheduler made an effort to keep the staff happy! For a while I simply told the scheduler I just wanted to be scheduled Mon., Wed., and Fri. but it didn't matter what shift I worked so that part didn't affect me.
As I said I don't think who ever does your scheduling cares? Unless you could get all your co-workers to unite and ask for the way the schedule is done to be changed. It would be ideal if you came up with a few concrete ideas, "solutions" instead of just complaints.
Scheduling staff is extremely hard.