My understanding is that the milk is tailored to the younger child. During pg, milk reverts back to colostrum, which is then more plentiful after delivery, becoming mature milk on the usual timetable.
If both infant and older child are nursing from both breasts, the milk remains more suited for the infant, though it certainly isn't "bad" for the older child, who likely is BFing significantly less often than the infant as well.
If mom assigns sides, e.g. infant feeds on right, older child on left, based on the frequency of feedings, duration of feedings (and ?suck strength) the left side (in this example) would revert back to a milk more suited to the older child's needs (more of a weaning milk, more concentrated).
However, it's not necessary to assign sides if tandem feeding; it's not dangerous for an older child to get milk suited for an infant.
