I do not know what dormer means, so please excuse me on that point.
Where is the Master technically located? Master bedrooms are typically a stand alone room with a bathroom. I've never heard of a Master being connected with a jack and jill.
My thoughts would be to have the 2 girls upstairs and share the jack and jill and you/hubby take the bedroom downstairs.
You and hubby need and deserve your privacy and it would not be ok with me to have play dates having access to my bed/bathroom.
When we built our home, our master is downstairs on the back side of the house and our daughter's bedroom was upstairs in the most furtherest area of the front side of the house. In the beginning (age 5) she was not too thrilled with being upstairs alone but she quickly adjusted and realized that she had the entire upstairs to herself and this was a perfect situation for her and when her friends were over.
Children can adapt to change and the must learn to do so because they will be required to adapt to change all their lives.