Change answer in child form

I have successfully linked my parent and child forms together, when I put in the client code all details pull across.

However, if I try to change the data it will not let me (it is set to read_only = false)

Should it be possible to change the linked answer in the child form? if so, how?

Thanks

You may need to use once(…) for your calculation. See ODK XForms Specification and for an example XLSForm Docs.