"Should DS be named as a beneficiary in DD’s application even though the trust is not intended for him?"
From how Metrobank word things it would appear that, yes, they require all beneficiaries to be named. Since the trust gives a class of beneficiaries (ie 'grandchildren') instead of just naming your DD then it appears that they do want the names of all the grandchildren.
Although the trust may not be intended for your DS, from what you have said he is also a potential beneficiary of that trust. As a result, Metrobank want his details as well.
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"They are named in each of their respective applications for trusts bank accounts."
"DD’s is discretionary and technically includes all GC"
It's not just a case of being "technically" - the trust does include all GC and the paperwork for the application needs to reflect that.
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"They are just saying there are no names beneficiaries even though DD’s name is in the allocation form and TRS."
I take it that was a typo and you mean application form to open a Metrobank account?
It may be that the solicitor and/or your parents incorrectly completed the form and left out the names of your DD and DS. Perhaps they just entered "grandchildren" instead of the names of both grandchildren and copies of their birth certificates if under 18?
If that is the case, then all that Metrobank are looking for are the names of the GC and copies of their ID if they are under 18.