I just read a story that named Echo as a child of Earth and Air. I loved that correlation. What other ideas can you all come up with for the “child” of other element combinations? Or are there some that are "standard?
Here are some of mine to start out:
Water and Air = a wispy little girl named Breath
Fire and Water = a ginger boy names Sizzle.
I would love to see more.
Earth and water, a pretty little girl named Muddy Steps.
Fire and Air = a cheeky boy named Smokey
Earth and Fire = a short-tempered blacksmith named Vulcan.
Oh wait - the Greeks beat me to it 
Such a fun creative exercise for mixing the elements! 
I am loving everyone’s suggestions so far! If we add in ‘Light’ and ‘Dark’ as elements, we could also get:
Light + Fire = a bright little child with electric yellow hair called ‘Zap’
Dark + Earth = a quiet child called ‘Shadow’
Really fun to think about! Thanks so much for this, @cbd 