Knowing my grandchild’s habit of throwing rocks, I’ll throw scissors.
Reasoning:
グー and チョキ in this case is rock and scissors respectively.
グーを出す to throw rock
グーを出す孫 rock throwing grandson
グーを出す孫の癖 the habit of my rock throwing grandson
グーを出す孫の癖知り knowing the habit of my rock throwing grandson
チョキを出す I’ll throw scissors
Knowing my grandchild
They’ll throw rock
So I’ll throw scissors
My first attempt at one of these 
Not very sure, but I understood it as something like that! I did omit explicitly saying the word “habit” though, so it’s not a very literal translation.
@Hubbit200 gets the credit for any likes I might receive for this submission. The sense of the translation is identical, I’m just riffing.
My grandchild / is prone to “rock” / so I show “scissors”
I love this topic 
My initial ideas for translations are SOO bad that I can’t post it here… It’s a bit like a RorschachTest and would reveal too much about my subconsciousness 
Me, too!
I still struggle mightily with the language, but I have a decent amount of cultural context from having spent so much time in Japan. Several of the coming entries are still completely mystifying to me, though, even if I can look up and understand every individual word.
I’ll try to pick one of the ones that puzzles me most tomorrow to see if we can work it out as a group.
It’s the same for me, maybe too much context or the wrong context.
For example I was convinced that グーを出す孫 is talking about the people who don’t like the vegetables (taking them out) in the Miso soup reading 孫 as 系 because of personal frustration with cooking for a family 
Usually I don’t misread Kanji if there is more context.
I think the “ingredients” word in 味噌汁の具 is pronounced ぐ, though, not ぐう / グー.
These things are really making me pay attention to every single syllable!
I’m not following how 系 relates to cooking, though. Oh! Did you mean 菜?
I don’t see 具 written in Kanji often, if I would think about it longer than probably I would recall that it is 具 and would also know it is written as ぐ, but people around me often use it when talking about how to make the family eat more vegetables (putting a lot of usually rejected things in the Miso soup anticipating that the taste of Miso and Dashi overrides everything else) and when they do it is pronounced like ぐーー but this might be a try to emphasize how much they stuff into the soup 
系 is often used to describe ‘that kind of people’ like in 草食系. So I thought it is talking about ‘that kind of people who are conservative about Miso soups and only accept it with a bit of Wakame and Tofu but definitely not asparagus and fancy mushrooms which is more to the like of most Japanese females who are fed up with eating the same limited five ingredients every single day’ which happens to exists in reality a lot actually 
(ask your wife…)
Gewwwwwww!
((edit cause I forgot the heading you’re supposed to use))
Tried to make it 5-7-5 in English
I know my grandchild
Will habitually throw rock
So I throw scissors
Or a shorter version:
Knowing my grandchild
Trends rock, → (or maybe “is keen on rock” or “leans toward rock”)
I throw scissors
Woah. Bonus points for 5-8-5. I like that first one a lot.
Thanks! Although I guess the syllable counting website I was using must not be working lol
I debated for a bit between “throw” or “show”, and I’m still not sure which is best! My first instinct was “show”, but “throw” seemed to be more common online. Even being a native English speaker I guess I haven’t played rock paper scissors enough to know which is best 
Your variation is pretty good, although maybe the middle sentence could use something extra? Such as “my grandchild / is prone to choose rock / so I throw scissors”?
Like you say though, all just variations of the same meaning!
@GearAid great translations! I like the first one, although not sure if habitually flows that smoothly spoken? I personally quite like your “leans toward rock” variation
One comment:
Maybe the following with quotation marks?
I know my grandchild
Habitually shows “rock”
So I show “scissors”
Reasoning:
Are senryu meant to be spoken aloud? Or just read?
Both I think?
There is a regular salaryman senryu competition, and I’ve seen them recited pretty often on TV shows and such.
Speaking of that competition: Here’s the info for the 2022 competition (35th year).
If anyone here submits an entry next year, please let us know! Looks like the first round of voting was on 3/18 so we’ve passed the deadline for this year.
Depends how you say the middle syllable of “habitually”. 
No translation attempt from me since I think it’s pretty much sorted, but I’ll take a stab at a less literal 5-7-5 just for fun:
Grandchild favors rock / I do know them very well / I must choose scissors
Trying to make a more poetic version.
My grandchild has a
Habit of throwing rock, so
I will throw scissors
Here’s hoping for a bunch of likes tonight else selecting a winner is gonna be tough tomorrow…
It’s five syllables for me, but I think it varies by accent, so I feel like either variation of the middle line is valid