Hi Tori,
as Julie says, it isn't carved in stone. I have my "basic" lens which I use for dog shows and most in-home "studio type" portraits (those where I bring my studio lights): it's the Canon 28-135 F3.5-5.6 IS. It's not fast and it's not as sharp as some of the L lenses but it works for me in a wide variety of settings.
I also have the 85 F1.8 which I use for candids and selective-focus portraits both in the studio/home and outdoors. I back that up with the 50 F1.4 when I want to work closer or have a wider field of vision.
And sometimes I use my 70-200 F4L for candids outdoors. At F4.0, it creates lovely isolation at shorter focal lengths.
If I were to advise on one "bread & butter" portrait lens, it would be something like the 28-135 IS, the 24-105 F4L IS or the 17-55 F2.8IS. All have excellent optical specs and the IS really helps if you have to throw toys with one hand while holding the camera in the other.
For what you're doing, the best single lens is a really good zoom. Later, you can think about acquiring "portrait lenses" like the 85 or the 50 or the 100 F2.0. IMHO.
Hope this helps,
Jim