Which two child elements can be in an XMPP message stanza?

As described under extended namespaces, a message stanza MAY contain any properly-namespaced child element.

In accordance with the default namespace declaration, by default a message stanza is qualified by the 'jabber:client' or 'jabber:server' namespace, which defines certain allowable children of message stanzas. If the message stanza is of type "error", it MUST include an child; for details, see [XMPP‑CORE]. Otherwise, the message stanza MAY contain any of the following child elements without an explicit namespace declaration: