We understand you are excited to have your son or daughter return home. Having been parents of missionaries ourselves, we can relate to that excitement. And we are so grateful for parents who support their missionary children by writing regularly.
But please remember that your child needs to stay focused on his or her mission up until the very last day. And when parents write about all the fun things they are going to do when the missionary returns, or how the parents can't wait for the missionary to meet his new niece/nephew/sister-in-law etc., or the exciting vacations they are planning on taking together, it really messes up the missionary.
I have visited with missionaries who are going home as early as March to as late as June who are struggling to stay focused due to letters from parents--particularly mothers. And when missionaries lose their focus, they get homesick, work less, are less obedient, and negatively influence their companions and districts.
So please be sensitive to that and help them "endure to the end" with an "eye single to the glory of God" by only writing things in letters that will help and inspire them.
Then, when your son or daughter returns home, they will truly return with honor, having been faithful and focused until the very end!
Thank you so very much!