Please help us help you and fill where relevant:
Your LimeSurvey version: Version 5.6.3
Own server or LimeSurvey hosting: Limesurvey cloud
Survey theme/template: Fruity theme
==================Tenim una enquesta a famílies sense tokens. We have a survey for families without tokens and we wouldlike that each family respnds about one of their children and that Lime would randomly make the selection. 
The situation: We have a first multiple response questions (previously we ask if they have children from 3-17 years old and how many) 1.  Of what aages are your children... (select all that apply)o 3      o 4      o 5      o 6     o 7    o 8      o 9      o 10    o 11    o 12    o 13    o 14     o 15    o 16    o 17    And we would like that from here, a message would come up that is somethign like:
From now on we will ask you about your child of X years of ageA partir d’ara et preguntarem pel teu fill/a de X anys:We have identified this work around in the forum - but we have not been able to get it to work. [url] forums.limesurvey.org/forum/can-i-do-thi...tiple-select-answers [/url]

We have created this test survey: 
depinstitut.limequery.org/767545?lang=ca  

Thank you and kind regards, 
Rachel

Hi, Rachel,
not surprising that the workaround fails.
This post is more than 8 years old and the solution for a different version of LimeSurvey

And it is always better to provide a lss export of these relevant questions.
This gives the change to include the solution and send back this lss file and you only need to import it.

Okay, to do this you do not need javascript. This is possible with only some equations. 

But first one remark.
Why do you use this long multiple question?
How do you deal with twins?

In my opinion an array with dynamically displayed rows might be better 
Or a multiple numerical question?
a. better Look&Feel
b. easier to select the child.

I created a sample survey with these options:
1. Multiple numerical input where the next field only is displayed if the previous is filled with an age
2. An array with dynamical rows
3. Your initial idea (You will see that the equations are much more complicated)

BUT: As I am not able to attach anything here you may have a look at this
mafosurvey.limequery.com/882211?lang=en&newtest=Y


Joffm

As I can't attach spmething

here a link to download 
Sample survey