You would probably find the easiest time right in the middle of the late dinner seating. Whenever I had to take DS to the restroom from Lumieres/Tritons/Royal court/palace the lobby area was always deserted.
It would be especially easy if you are all doing early seatings, just skip one of the shows that evening. If you are doing late then you would have to coordinate a specific time. Probably like a 30 minutes after seating so you are all somewhere between apps and the main course. Below is a pic I got of a group getting a photo during one late dinner.
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