There is no clearly defined range for each of those generation labels and there is some overlap. The best answer is GenX are the children of Boomers and Millenials are the children of GenX.
I was born in 1980 so according to demographers I can be considered either one. Usually someone in my age group is considered part of GenX demographic if their parents were baby boomers and a Millenial if their parents were young GenX'ers.
GenX'ers are kids born in the late 60's until the early 80's and Millenials Early/mid 80's to the mid 90's. If you're in the cusp they tend to base the demographic on what generation your parents were in.