12 month old will be easier to adjust than 3 year old. So if doing transition, it more important for older child.
I not do transition unless parent is paying for it. They start a week early and pay full time and phase in that way.
I tend to issue my schedule long before start date and some parent mirror mine before child start here. If baby schedule different, I mirror their's for a week and start bringing into mine. But lunch/naps not changed, only routine. So if baby has bottle before nap, I do that here or if they have when wake, I do that here.