Brazil and Argentina don't have any serious interest to join the Entente (and sacrifice their soldiers for London), nor to join the Alliance (at the risk of losing valuable trade with Britain... and angering the USA). Only neutrality makes any sense for them.
Well, maybe if the USA have already joined the war, and Germans are sinking neutral ships, they might join the Entente (at little risk as an invasion of South America by Eurasian powers is absolutely impossible). They'd probably deploy a few ships to help the Entente patrol the ocean, maybe a few thousand soldiers, and that's it.
Or conversely, if Britain and France just have fallen, they might swiftly take over Guianas and Malvinas (preempting a German takeover).
So either full neutrality or low-level involvement.