With so many schools having full day kindergarten there is no money in taking the child for just before/after. Around here kids up to grade 2 have to have an adult meet the bus so it also means taking the group out for the few dollars you would get.
In this case I would simply explain to the parent that since you won't be able to do the school trips that it doesn't make sense to switch to PT. Given the timeline I probably would advertise to fill the space for September fulltime. That gives mom time to recover and get a good routine at home and for the baby to get a few months old. Let child switch to pt for summer if they want to gradually move to time at home.