How "Playing Peek-A-Boo raises fun" works
This has to be the simplest mod in my repertoire.
As always, I start by seeing what's the interaction name using Lumpinou's Sim Info.

I'm not sure what social_Infant_PeekaBoo_Friendly_AlwaysOn_STC_Succeed is, and, to be honest, I didn't feel like looking it up! I just wanted a quick solution so my adult Sims feel better after interacting with their own babies.
I then go to the TDESC Builder, and go directly to "File -> Import EA Tuning".

In the search box, I type social_Infant_PeekABoo_Success. And then I click Import & Edit.

And now, since I have some experience and mods under my belt, now I know exactly what I'm looking for! We want something to happen after they play with their babies. This is a loot!
I scroll directly to the loot_list section, and I see it only has 162659<!--loot_Commodity_Culling_GhostAdd_target-->. If I had to guess, this makes it so the ghost is less likely to be culled. But it doesn't give give fun to the adult. In fact, it doesn't seem to do anything else.

So all we need to do I just add loot_Commodity_Motive_Fun_Gain_Small. To do this, we click on the + icon inside loot_list.

We'll get an empty box that says action.

There, click on the clip. When you hover on that icon, it'll say Select Reference.

Once you click on it, you can look up loot AND fun if you want to see what other kinds of loots exist that work with the fun need. There are a lot, so I'm just going to look directly for loot_Commodity_Motive_Fun_Gain and click on the first one.

Now, the mod should look a little like the image below.

And... that's it. That's the entire mod! Now our adult Sims earn a small amount of Fun after playing with their infants. All you have to do is go to File -> Download Package, and save that .package in your mods folder.
Unfortunately, I can't remember how I learned about loot_Commodity_Motive_Fun_Gain_Small. I'm sorry! Other than that, I hope you learned how loot works.