Morning KJC,
I have a 67 M20F and we absolutely love it! My wife and I have a 14 year old and a 10 year old. They fit just fine and with the tanks topped off we even have room, and W&B, for a small suitcase. Once we climb in and slide the front seats forward the kids have more room than the front seat so legroom is not an issue for the kids. As the kids grow we'll dip into that reserve a bit and then fly with less gas.
As it is my longest flight is a 5.5 and still had over an hour of range at touchdown, but we usually keep the flights shorter so the kids can hop out and get a break.
I am a huge fan of the mooney, it is a very stable IFR platform and really easy to fly, once you figure out how to slow it down. If you can find an F with the speed mods then you'll see speeds closer to a J without having to pay for the J.
If you have any questions then let me know. I'm based on the east coast out of KHEF.
Sheriff