Traveling PDX-OAK-AUS with my family this am and had two different boarding experiences...
My wife broke her ankle on our trip and is in a cast and crutches and was given the courtesy of pre boarding in PDX - we were travelling with our 8-5-3 year old children.
In PDX they let us all board together as I (father) was the Sherpa/moms helper/etc... it all worked out fine.
However, in OAK the agent told us that my wife should take our oldest child during pre boarding and I should wait for family boarding with the younger 2!? He ultimately let us board together, but, left us all feeling like we were taking advantage of the situation... I could buy that if my daughter was maybe 12 or 13, but, these are young children...
The OAK agent was actually following the policy, but some situations like yours require some flexibility. Here's the applicable section of the preboarding policy:
We will allow one travel companion to act as an “attendant” and preboard with a Customer with a disability. In most cases, the Customer requires assistance from only one other person, and any additional family members or friends are asked to board with their assigned group.
So, the agent saying only one other person can go with your wife is technically correct, but the point of that second person is to assist the disabled customer. This would mean your 8 year old would be considered the "attendant" which, speaking from experience with 8 year olds, is probably not the best plan, while you and the other kids wait for family boarding. In the end it sounds like the agent realized the best situation was to allow you to be the helper which then means your entire family preboards. Thankfully common sense prevailed.