I'm not sure any explanation is possible, but I'm going to do my best.
I'm reading Bambi to the boys. It's my first time through, so I'm proceeding cautiously. We reached the part where the human hunters have invaded the forest, killing everything in sight. Poor Gobo, frail and exhausted, has collapsed in the snow, his mother forced to abandon him to his fate to protect herself and his sister, Faline. After finding him lying there helpless, Bambi pauses, but cannot help his friend, and so ultimately dashes away at Gobo's insistence.
"Why doesn't Bambi help Gobo?". That's Thomas asking.
"What do you think Bambi could do to help Gobo?" I just love putting the ball back in their court, and making them think about it.
"He could get his mom". Thomas still thinks mom can fix everything.
"If Bambi could find his mom and Gobo's mom, what do you think the moms could do for Gobo?"
"They could help him run".
This quickly moves into a conversation about how from the moment of birth, young deer must be able to stand and move on their own, or risk being killed by predators. Deer don't have hands, and can't carry their young in the way that humans, primates, and other animals may be able to do, so their young have to be mobile almost immediately. Gobo's mom can't just pick him up and carry him to safety. She is forced to leave him lying there, helpless, to be murdered by the humans. What a horrific burden faced by Gobo's mother. What a horrific dilemma faced by a child listening to this story for the first time.
After concluding the chapter, Thomas finds his own way of dealing with this emotional turmoil. He runs upstairs and dons the kangaroo costume. He then goes through considerable machinations to insert Michael (4) into his "pouch", thereby assuring that his youngest brother will never be left floundering helpless in the snow to meet an untimely and horrific end. It's a game for the boys. But, also a way to work through and resolve an emotional dilemma that is almost beyond their compreshension or ability to process and resolve.
"Acting out" would be an apt term. Working through something in a very physical manner, to come to an emotional wrap-up that they're able to "sit with". It's another one of those "boy things" that I only vaguely understand, but try to recognize and embrace as it presents itself.
It seems to have worked in this case. Everyone is smiling.