It sounds like the dog wishes to be higher in pack ranking than your daughter.
This is a very strong minded breed that was developed to boss cows around so the fact that they are bossy is bred into them and of course any dog will try to be as high in the pack rankings as it is allowed to be.
I would strongly recommend a basic obedience class of a positive type as a good idea for the dog. http://www.apdt.com/ may help you locate one near you.
For around the house I would suggest new rules for dogs and humans so that the pup knows she is not the leader of the pack or boss over your youngest child.
The nothing in life is free program can be a help with this change in roles with out any violence. You can read about this here
Dogs are perfectly happy in being followers but if left on their own without leadership they can get awfully bossy to the point of growling or snapping if their 'rules' are not followed.
Clicker trainng is something your daughter is old enough to do along with you. You can see some good videos on clicker training as you scroll down past the ads on this page
Once the dog understands the clicker training you can even use it to tell the dog yes that is right when she is nice to Megan and encourage correct behavior.
If good things happen to your pup when Megan is around then the pup should have an attitude change about her being nearby or petting her etc.
But she has to be good first and rewarded second. Right now you are rewarding her to try and stop her bad behavior which isn't quite the same thing in her mind.
Hope this helps you!