Translation:There is a table in between the boy and the girl.
You guys better familiarize yourselves with postpositions. :) Hungarian has no prepositions, relations are marked right after the objects, not in front of them (most "cases" can be seen as postpositions that have turned into suffixes).
Having said that, I don't see how "the boy and the girl between" would be any more difficult than "between the boy and the girl".
Van egy asztal = there is table. (Van can be both "there is" or "is", depending on the sentence, but it the sentence starts with Van egy, it is very likely that we deal with a There is a construction. )
a fiú és a lány között = between the boy and the girl. között is a postposition, so it is placed after the thing it refers to, unlike in English, there between is placed before.
I'm a native English speaker, so I'll see if I can help... The 'in' isn't absolutely necessary in the sense that the sentence is still grammatical and intelligible without it. However, I wouldn't call the 'in' unnecessary, either. I sat and thought about this awhile and how to explain it. It seems that we use 'extra' directional words in front of prepositions in some cases to specify, ie to give more information, about what we want to say with the prepositional phrase. Also there seems to be a sense that 'in' in this context demonstrates that we are talking about a concrete and not an abstract statement. For example, say that the girl and the boy have been having a disagreement and are no longer speaking to each other. We can say they have a (metaphorical) wall between them. 'There is a wall between the boy and the girl'. Using 'in' here does not sound natural. The wall isn't really there physically. Okay, if on the other hand, the girl and boy are sitting each on opposite sides of the wall between their yards, you could say - 'There is a wall IN between the boy and girl' - meaning that their is an actual wall located between them. You could also think 'in the between', although using 'the' in this case would not be correct. One more example - like I mentioned earlier, you can use an 'extra' word to help describe what the prepositonal phrase (ie the preposition plus its noun and modifiers) - for example: The boy and girl sit down at the table. This is perfectly correct as 'The boy and girl sit at the table', but you get less information. I hope that all this helps.