The assumption implicit in your argument is that the relationship is equal (as in the case of a boyfriend/girlfriend). I'm not sure that same assumption holds for a parent and a child, as much as the media tells you otherwise.
The point of the example was merely to rebut the idea that a parent's prerogative to choose destination doesn't mean there's no transgression. Finding some way that it differs from the parent-child relationship isn't relevant unless you explain why the asymmetry would imply there was no transgression.
Children learn their own forms of manipulative behavior at an early age(e.g. cry to get attention or what you want), but with less logical understanding of their own wants/needs. Put children on the same level, and hold the child to the same moral standard you're trying to hold parents to and they fail completely at it. It's not a matter of the parent manipulating the innocent child, but rather the parent diverting around the child's own manipulative behaviors. Perhaps no one should be manipulative ever, but go ahead and try explaining that to someone with the cognitive and emotional maturity of a 4 year old.
In an adult-adult relationship, the two parties should be equals, with similar cognitive and emotional maturity. They should treat each other as such. This isn't always the case. Even loving seemingly healthy relationships may involve some form of manipulative behavior on both sides.
Transgressions occur all the time, in all relationships, you have you to use judgement to figure out which ones get a pass, and which ones don't.