What can I use to clear coat tempera or acrylic paintings for a strong glossy look? I presume the suggested coating might vary between tempera and acrylic paint? So two different products is fine by me. I just need something to give my paintings an extremely glossy surface. The coating should be as transparent and color-less as possible so the underlying colors show well.

What could I use for this?

Oh, also, I might like to do the same thing for latex paint in the future. Any suggestions on that as well?
You can use glossy acrylic medium. Acrylic medium comes in different bodies (thicknesses) and the heavier the body the deeper the layer will be. There is also acrylic varnish which also offers more UV protection and lets you clean the painting later but will have less body and is potentially more toxic.  

Use a soft wide brush when applying and optionally add a leveling agent to help make it smoother and prevent brush marks. 

The difference between tempera and acrylic paint is that tempera may remain water soluble after drying in which case it may bleed into anything you coat it with. Test it and if it does bleed you may want to use a fixative before applying the gloss coat as addressed by this question: What can I expect from a fixative? Though with fixatives you run the risk of discoloration and any glossy coating is going to darken (ie. deepen) the colors.