Thank you for posting the extensive info and experience with this T slot material. I attempted a build using other approaches and came to a dead end, so I am back considering a T Slot approach. My goal is similar - ability to deal with 5 ft wide sheet material.
I am glad to see how your joints have turned out, because frankly it settled a fear I had about how well butt joints would work out (if the cuts are not accurate enough ) Thank you also for the extensive comments about material selection advantages / challenges.
I am still not entirely sure how to deal with a corner joint that has an upright post, an X and Y directed T Slot frame all meeting at the same place. I guess it can be accomplished with gussets, although it would be handy if it could be done with screws through the frame. I can't see how this can work though since the screws would likely interfere.
Good luck with the twins - it gets easier after about 1 year, but hiring a "mother's helper" can be a real useful thing to do.