Not sure if you're trying to make the same thing, but sounds similar to the store object:
https://www.youtube.com/watch?v=5K4nus5qRfA
I believe the ITUN file for objects are tied to objects of the same class. For swing I assume it would look something like
<Interaction name="Sims3.Gameplay.Objects.Environment.SwingSet+Swing+Definition" />
<Object name="Sims3.Gameplay.Objects.Environment.SwingSet" />
...
<BaseAvailability>
<AgeSpeciesAvail AgeSpeciesValue="C,T,Y,A" />
Just throwing out thoughts, I'm thinking the way you would do this is maybe create a new scripted object which is a subclass of Sims3.Gameplay.Objects.Environment.SwingSet (scripting mod - though technically if you are just reusing the code for the original swing object, you don't need to write any new code other than declaring the subclass), call it ToddlerSwingSet. Then create an ITUN file for each interaction that you only want for Toddler, and replace the age availability with just "P".
Alternatively you could just override the ITUN files for the original swing object, add P to the list, then create a bunch of new animations that start with the p_/ p2o prefix for the existing swing animations, but sound like you want it exclusively for toddlers so this might not be what you want, but could be a good initial phase to test out against the original swing set.