hmm
1) Come on back in - return fromsomwhere outside of where the speaker is
2) from out of down under - get out from underneath whatever it is you are under
3) there - the �whatever � that is referred to in 2)
so, in clear English it could be...
Come out from under that thing (from out of down under there) & return to the class/ to the house/ to me / ... ( �come on back in� has been given a point of reference for purposes of providing an example. The location would be cued by the physical location of the speaker, so stating a location would be optional)
If there was going to be a reference to Australia then generally it is commonly one word �Downunder� & used as a proper noun. Sometimes you will see separate words, but in that case it should still be a proper noun phrase �Down Under�
Regards,
AB