I would put the Regent behind the Driver in the captain chair. The MA would go in the passenger captains chair with LATCH but moved to the center of the vehicle. The Booster would go in the 3rd row in the center (I think this position has shoulder belts?).
Reasoning:
The Regent would be a seatbelt install and I don't want the seatbelt to interfere (sp?) with anyone getting to the third row, hence, behind the driver.
The Marathon goes in the center because that's the only middle row spot left, and I want clear access to get to the third row, hence, captains chair moved to the middle position.
The reason I would put the MA and the Regent next to each other... the 7 year old is probably in school most of the time, so I'd want the other two closest to me.
I haven't invesitgated the Sienna for RF tether points, but the seat track or belt stalk is a good first place to look.
Edited to add: This would probably only work if the Sienna has two doors... I can't remember if it does or not? If it doesn't have two doors...
The MA would go RF behind the Driver, the Regent would go in the middle row, center position captains chair with LATCH (if he's still light enough) and the booster in the 3rd row. Same reasoning... easiest way to load kids and keep the younger two closest to me.
If the 4 yr old is not light enough for a LATCH install, it would stay in the outboard passenger captains chair and get seatbelted in or go center... whichever is easier for the 7 year old to climb around and get to the third row.
Wow, did I confuse you with all those configurations? I think I confused myself. It's really what's going to work for her... how many times/who does she load/unload?