Being a local here in BA I can tell you that we do not celebrate Halloween, but I do know that in places like "The Alamo Bar", "Fusion Bar", "The pub Crawl", they have Halloween partys. They may not be for children, but you'll have a great time there. And hey, you can even ask them if they know any place for kids to hang out during Halloween.
I have no idea where there is a Halloween bash for kids but I'd love to find out more.My little fella is only 1 n a half but getting him dressed up n a couple of photos would be worth the search.Maybe there's some other expats with nippers or even without who might be interested in organizing something fun...I'll do a bit of checking and we can keep looking.
Welcome to Argentina!
If you need some food with Halloween decoration, please let me know. I bake American, Argentine and European sweet stuff, ie. cup cakes, cheese cake, lemon pie, brownies, etc etc.