Wedding Guest Confusion

I'm trying not be a Bridezilla and sometimes have to ask myself several times if I'm overreacting because some of the questions I'm asked I can't seem to wrap my brain around. For starters, we're having a destination wedding and have, so far, only sent out Save the Dates. One of the guests said they couldn't make it and offered to give their Save the Date to a couple who did not receive a Save the Date and we had no intention of inviting. Asked if they can hand off their Save the Date unless we have "someone else in mind" to replace her. I thought if a guest cannot make it, the bride and groom would then decide who, if anyone, would replace that seat. I don't know how to politely reply to this "request". It's an aunt so I have to be very careful how I respond and I just don't know how to. Another guest asked if our children were attending the wedding so they can bring their children. Our children are OUR children and are expected to attend their parents' wedding.