I would recommend splitting the PNR for 2 people to get the upgrade if you are fine not sitting all together as a family. If you do get upgraded, you would be able to check more than 4 bags (I think 6). In regards to the lounge, you would get access for 2 people, however, if your wife is an AU or has her own Amex plat, she would get in as well for free, meaning you would only need to pay $29 for one child. When it comes to visiting between cabins, I disagree respectfully with MSPeconomist. While IMO you shouldn't bring coach passengers up to First, I would be comfortable going back to check on the other two passengers as long as you aren't blocking the aisle. Safe travels!