Idiomatically, the term "agreed upon" is correct. Preferred, even. It's a tad formal and the phrase "agreed on" is also unambiguous. There is slight ambiguity when the boy delivers "the agreed resolution" in that it suggests mainly that the resolution was considered final by all concerned plus a small ambiguity that the resolution was one of agreement as to its internal terms.
As an "agreed upon" or "agreed on" resolution, the only agreement is that the final submitted resolution was agreed to, not that its internal particulars were made up of agreements, point by point.
If I were using the idea in poetry, "agreed resolution" might be better for the meter of the line. We need a solution to the agreed resolution.