"We like to think of [insert name of youngest child] as our middle child."
Congrats on number 7. We have number 6 in the chute. :-)
Another reply you can use when people ask the always inappropriate question "Are you done?" is: "Why do you ask?" Of course you do this with a smile, and I would only use this if the person was being obnoxious about it. Thankfully neither my wife nor I have had to use it. Most of the time when people ask this question they are being friendly and not really aware of how out of line this question is. I often just shrug my shoulders and say "Who knows" while smiling. I then usually go on to make some joke about having a basketball team or now with 6 that I don't have to play goalie anymore. Usually this causes a smile and the whole uncomfortable moment passes.