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in addition to the fact that they did not really add all that much advantage in terms of energy
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jimbob, I'm gonna disagree with you there. Initially, off hand, my thought is: 6.8SPC bullets are heavier and have better ballistic coefficients. Again, off the cuff here. But how much slower are they going compared to a 5.56 was something I did not know. So I looked it up.
The second piece of the story is something that (can be) slightly debated: using kinetic energy (KE) as a unit of measure of effectiveness of a round. Not here to debate that, but it gives an easily comparable number to use, so we'll use it knowing that shot placement is paramount. Chuck Hawks had a good article that really baselined 800ft/lbs of energy minimum for killing a deer cleanly, so we'll use that number.
On Hornady's website they list "appropriate" uses for their bullets, so picking a bullet in 6.8SPC and a bullet in 5.56 that are both 'recommended' for deer and then matching barrel length gives us 2 bullets out of a 16" rifle, useful as you wanted a rifle that was still handy for your daughter. The 6.8SPC retains our minimum energy out to, just under, 400yds. Whereas the 5.56 only retains that energy to just under 200yds. I'd say that's pretty significant and that moving up to an AR10 gives you, potentially, more range (didn't look that one up), however you had to switch to an entire new platform.
Not saying that you didn't pick the best option for your situation, just that I feel that the 6.8SPC DOES give an appreciable gain over a 5.56 without having to go to the AR10.