Why do you keep saying that letting pax with small kids on first is not for the benefit of those pax but actually for the benefit of everyone else? I have a small kid and it is great to be able to get on and get settled without having to wait in the aisle for everyone else to sort out their bags. I also travel a lot without the little one and it is absolutely no benefit to me when the pax with small kids board first.
Bottom line is that this is your fault for not paying for the service you wanted or being the first in the queue with everyone else. It is in option you have, if you decide to not take that option then you should expect not to have it.