Translation:The parents are going to write a letter.
Just a general comment about this whole "πρόκειται" thing. I think it is being presented as a form of future tense that is as common as "be going to", which is not accurate. It is definitely less common and not really a grammaticalised construction of the future tense (i.e. unlike "θα + verb"). It is more of an equivalent to"be about to" than "be going to".
Thanks for this insight @elliest_5. I was wondering about this phrase too, as I usually use "Θα" instead in practice. So it sounds like this translation should be "The parents are about to write a letter" ?