Both the SB400 and the SB600 work fine with my D5100. You don't need another flash. Either a setting is set wrong or your D5100 has a problem. Check into it.
I have no problem at all using the D5100. I find changing settings pretty easy. While I change ISO frequently the other settings not so much. Pressing the"i" button and scrolling down is pretty quick. Just spend a little more time with it and it pretty much becomes second nature.
I use my D5100 for nature, macro, birds and as a body for my non-AI manual focus lenses. Except for infrequent family gatherings I seldom take photos of people. I don't have any little ones around the house. Even my grand kids are in college.