I think this question is a good illustration of how you have to think about translation functionally, at least in a case like this, rather than just translate the words.

Yes, if you need English words for "solana" and "umbría", you'll have to use "south-facing / north-facing slope" or "sunny / shady slope". We don't have any comparable terms to the Spanish ones.

But if you just replace one with the other, you'll end up with either this:

"The slope that receives more sunlight is called the south-facing slope and the one that receives less is called the north-facing slope."

or else this:

"The slope that receives more sunlight is called the sunny slope and the one that receives less is called the shady slope."

The second of these is frankly a bit ridiculous, isn't it? Talk about stating the obvious...

The first is more promising, but it doesn't really work in that form. It's the wrong way round. It's not a question of being "called" the south-facing slope; it "is" the south-facing slope.

The point is that the function of the Spanish sentence is to teach the children a pair of terms. We don't have such terms, so the point of the sentence is lost. We will have to use it to teach something else. Not that the sunny side is sunny, which is not teaching them anything, but that the sunny side faces south.

So this would work, up to a point:

"The slope that receives more sunlight is the south-facing slope and the one that receives less is the north-facing slope."

All the same, I don't think a teacher would actually say that to the class. I think he/she would say:

"The slope that faces south receives more sunlight, and the slope that faces north receives less."

That, I think, is beginning to be usable in class. It's a bit simple, but OK for young primary.
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A possible further step with older children might be to adapt this to teach a useful geographical term: aspect. A south-facing slope is said to have a southerly aspect. So you could extend it, by saying something like this:

"If a slope faces south, we say it has a southerly aspect, and if it faces north it has a northerly aspect. A slope with a southerly aspect receives more sunlight and a slope with a northerly aspect receives less."

This is going beyond translation, and is probably not a good idea in an exercise, but if it were a professional assignment I might be tempted to discuss this idea with the client.

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Note added at 1 hr (2014-11-20 14:37:54 GMT)
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All this, of course, is assuming we are in the northern hemisphere. Better make sure the book's not going to be used in Australia :)