There are show slots available when you board. Use the app once you are connected to ship WiFi to try to book or look at the planner for where you can go book in person.
My Time Dining has two lines: one with a reservation and one without. You can always get in the no reservation line though on our sailing MTD didn’t open until 6.
Also in the planner is an open house for the nursery services (babies and tots). If you want to leave your child with them it costs $6 an hour and reservations are definitely recommended as they can only watch so many kids. You have to schedule with them in person or maybe with a phone call after you initially register. Inside the kids area there is also a playroom with a mat, toys, and play houses if you just want to take the kid up to play a bit while you watch them. We did that with our son a bit each day.