Hey, just randomly saw this again and thought about another option.
You could do Count Children then Int Op to subtract 1 and that would be the last index.
I know you're looking for a single action that would do this but I don't think there is one.
I run in to this a lot and I'm not proficient in C# so I end up coming here for advise and/or just finding a way to get my desired result and moving on (usually the latter).
Otherwise, I would advise experimenting with the Custom Action Wizard to make your own.
Lots of people do this, post their creations here on the forum, and they sometimes end up in future updates or on the Ecosystem for everyone to use.
It's kind of a really rad part of this community.
https://hutonggames.fogbugz.com/default.asp?W166Hope you're project is coming along well
Cheers,
Jim