Translation:The children were looking for the cake in the fridge, but it was not there.
So.. "keresték a süteményt" is "(they) were looking for the cake." If I put "(they) looked for the cake" here, is that wrong?
Is the "aspect" wrong? In order to really mean "(they) looked for the cake", is that when a "meg- or ki- or el-" preverb would enter into the equation?? like: "megkeresték"?
Do you mean simple past vs. progressive? Sorry, I am just a little confused as to what exactly you are asking. But I guess that's what it is.
So, in that case, I would say both are valid. Yo don't need the "meg-" here to make that difference.
In fact, adding "meg-" here carries a different meaning. It will mean that they found it!
"Megkeresni" is "to look for and find". To locate.
Now that's a real completed/perfective action! :)
No, "felkeresték" is yet another thing, it means that they visited somebody. :) Some of these preverbs give idiomatic meanings to verbs, so you never know. English does the same all the time with all those verbs and pospositions: look up, put up, put off, give in, etc. etc.
Hungarian itself has only one kind of tense. Simple tense. So, if it is important to express the process of some action, we have to create the context for it. For example, while they were looking for the cake, something happened. "Amíg" keresték a süteményt, valami történt.
But, in this particular case, I don't see much difference between the simple and the continuous. It does not make any difference to me. Does it to you?
You can experiment with the word order, and you can achieve some minor differences, some of them may feel more progressive, but it is not really significant.
"A gyerekek keresték a süteményt a hűtőben, de nem ott volt." - This is maybe a tiny bit more progressive. Emphasis on "keresték".