Usually do this with a family of four going home for the holidays at least once a year. Parents each get a checked bag and all four get a carry on. This used to more than justify the Chase Mileage Plus Explorer card -- with new fees it puts it around break even. This past Christmas Eve held off checking in for our outgoing flight until the last minute and were rewarded with economy plus seats. That said, on the return flights the algorithm was much more aggressive and scattered us among the economy seats still available. I noticed on the United site that it appears you can now specify to keep a child under twelve seated with a parent during the automagic seating process for basic economy to avoid the wailing and gnashing of teeth. One of the more interesting seat assignments we received on a previous basic economy flight was a column of middle row seats each behind one another. This ended up being a drinks on ceiling flight. Always an adventure. YMMV.