Welcome! We're in the Twin Cities as well!
I know that pinatas are part of Guatemalan birthday traditions; the little girl we sponsor in Guat had her birthday on Tuesday and for the second year in a row we also "sponsored" her party (which included a pinata! The kids love them!)
Once your boys are home, you can find pinatas at just about every "party" store in town, but for some that are more authentic, try El Burrito Mercado on Concord Street in St Paul--they have *everything* (including the most beautiful jalapenos I've ever seen)
Another tradition I've heard of is offering the birthday child a "Tres Leches" (three milks) cake. I know various recipes have been discussed on this forum, so you could run a search for it.