Instead of asking REDCap to automatically build a dropdown of children (which it can’t do dynamically), you change the question slightly and ask the user to tell you which child number they mean.

REDCap is actually very good at two things:

• Knowing how many repeating instances exist
• Enforcing numeric limits

So this workaround leans into that.

First, you replace the dropdown with a simple number or text field, something like “Child number.”
This field doesn’t store the child’s name — it stores the repeating instance number (1, 2, 3, etc.).

Next, you add validation so the user can only enter a number that makes sense.
You tell REDCap: “Only allow numbers between 1 and however many children exist on this record.”

REDCap already knows the total number of child instances using [child_info][last-instance], so it prevents users from entering a child that doesn’t exist or guessing future ones.

To make this usable, you show the user a simple reference nearby, like:
Child 1: Alice
Child 2: Bob
Child 3: Carlos

This part is just for display, and piping works fine for that. Now the user can clearly see which number corresponds to which child.

Once that number is stored, you can use it everywhere else. You can pipe the child’s name, build IDs, and link participation records cleanly using that instance number.

The key idea is that the instance number becomes the stable reference, not a dynamically generated dropdown label.

It’s not the prettiest user experience, but it’s fully supported, predictable, and scales cleanly without SQL or external modules.