If you want to have many children, but counting on the older ones to help out with the younger ones, maybe don’t have many children.
Older kids are not there to bear the responsibility over their younger siblings. The fact that they are the oldest shouldn’t affect their own childhood, and they shouldn’t be expected to ‘help out’ just because they are there. They are kids just as much as their younger siblings, and to put is as simple as possible–it’s just not fair.
No one can blame those older kids when they finally stand up to their parents in that matter. They shouldn’t have to put up with always being on call as the family babysitter, and they shouldn’t keep quiet about it either. When they stand up to their parents and say «Hey, it’s actually not my responsibility to take care of your kids», they are the ones who teach their parents a thing or two about accountability, and that is a lesson everyone should learn.