Not at all!! Your job was to find the best resort at the best price. If any guest chooses to deviate from your plans, that's their choice. Let your aunt knock herself out trying find a better price than a group rate, lol!!
You'll find that some people will try to book last minute, hoping to get a better price through a discount travel site. That's essentially the same concept -- it's their choice to do so and it's not your responsibility to find them a "better price".
Don't worry about people not enjoying themselves if they bring their children. If you chose a kid-friendly resort then they will all be just fine. Plus, people with children have the choice to leave them home with a relative or sitter, or bring with, in which case they know what they're in for. The important point being you shouldn't feel responsible for ensuring people with children have a good time!
Good luck!