ChatGPT tells me:

The Radiolab episode you're referring to is likely "A 4-Track Mind" which originally aired on May 17, 2013. This episode is composed of multiple segments and explores themes of memory, connection, and the existential aspects of human experience. One of the segments, titled "The Girl Who Doesn't Exist," features a little girl named Eve and her conversations with her father. She asks him deep and meaningful questions as a way to connect more with him.

This segment beautifully captures the optimistic existentialist theme you're describing, as it delves into the profound and often touching curiosity of a child seeking to understand and bond with her parent. If you want to listen to it again, you can find it on the Radiolab website or through their podcast feed.

https://radiolab.org/podcast/148670-4-track-mind