Can a child's device be controlled by multiple family members? Surprisingly, not all children live solely with one guardian. Is there any way that a child's device can be controlled by more than one adult?

The situation: My ex-wife and I have 50/50 custody of our kids, and our 13-year-old has an iPhone SE. He has an Apple ID linked to my own and Family Sharing is set up for the two of us. To help reinforce healthy phone habits, I have all extraneous apps (Instagram, Youtube, etc.) restricted by default, and only turn them on when he is specifically allowed to be using them. However, he's at his mother's place half the time, so if he wants to use his phone she has to contact me to allow them, then again when time's up. Fortunately, we are co-parenting fairly well and have very open communication when it comes to the kids, so it's not a big deal in that regard, but it's extremely inconvenient, and I'm not always able to stop what I'm doing and turn on his apps immediately.

Is it possible to associate his Apple ID with hers and add him to her Family Sharing group as well? Alternatively, if I add her as a family member to my group, can I give her the same full parental control access as me?
Your child's account can only be associated with one Family Sharing group at a time. The best solution would be to add your ex-wife to your Family Sharing. There's an option to mark an account as a Parent/Guardian, which gives that person authority to approve requests from a Child account in the Family.