A couple of points to note here. Firstly, the whole '85mm best portrait focal length' is a load of rubbish. The fact is that there is no best focal length for portraits, it all depends on each individual shot. As there are so many variables, it's impossible for one lens to stand out above all others.
The notion stems from studio photographers who, given the restricted room of a studio, would prefer to use an 85mm lens for head shots as the working distance gives a flattering perspective to the features of the subject.
Note that the key phrase is working distance and not focal length. Too many photographers think that you can't use a wide angle lens as a portrait lens as it distorts the features too much. The truth is that if your working distance is the same, the perspective is exactly the same no matter what focal length you use. Thus, wide angles make fantastic portrait lenses is used in the right way.
One of the advantages of the 85mm is that most variants have a fast maximum aperture, and are therefore capable of reproducing very narrow depth of field, defocus the background completely and focus attention on the eyes. Two things about this, firstly it is one style amongst a host of others and therefore can be overdone just as any other style can be. The second point is that any camera with a fast maximum aperture will allow you to narrow the depth of field down to Rizla thin proportions depending on working distance and sensor size.
That said, in your position, with a cropped sensor and a 70-200 already in the bag, my first preference would be a 24-70/2.8 for speed and flexibility. The whole narrow depth of field style is fine, but unless you're shooting head on you'll find that you'll have one eye in focus and the other out (in fact I was shooting head on with my 200mm f/2 last week and getting shots with one eye in focus and the other slightly out, the depth of field is that narrow) so you'll be stopping down anyway just to keep focus sharp across both eyes. A zoom is far more practical too, especially with children and allows you to maintain your working distance while changing the angle of view. One time when 'zooming with your feet' can be troublesome is when shooting portraits because that's what changes your perspective and can lead to unflattering distortion.