Not bizarre if you have been in that situation.
OP can't pick child up from class/teacher due to disability.
There are 4 children involved.
7 & 10yo in school 12yo picking up the 7yo, and a further dc that the OP has to stay with outside the school.
10yo is yr 6, so likely released and allowed to just go/find parent/walk home.
7yo needs to be handed to/collected by an adult. Previously school was ok letting the 12yo do it.
the 10yo was never part of the equation with picking up the 7yo.
Reading between the lines, the other DC is potentially disabled, and can't be bought onto the school grounds during collection.. my guess is likely autism or some health issue that means the crowds/noise will distress them/cause them problems, and also why the 12yo can't be left to supervise them while OP gets the 7yo.
By agreeing that the 10yo can get the 7yo from class, and both meet OP in reception 15 mins early, it means that OP, and her other DC, are not being subjected to the chaos that is let-out time, either in school, or on the playground, and will be clear of the school and grounds by the time everyone else is let out.
The fact that the school have agreed to this accommodation in the meeting the OP has clearly had today (hence asking for advice this morning) shows the OP's situation warranted some adjustment to help her.
This was something i used to do with my ds who has ASD, he used to arrive 15 mins late, and leave 15 mins early because he couldn't cope with the noise and hubbub, and by default, so did his younger DSis so i could drop/take both at the same time.
There is nothing bizarre or odd about any of it.