The TRIAL JUDGE (i.e., the trial judge).
BUT, WHY did the trial court decide the facts in this case instead of a jury?
Because, there was no jury in this case. The opinion does not explain WHY there was not a jury. However, in most civil cases (as opposed to criminal cases) the right to a jury trial is not absolute. Instead, if either party wants the case to be tried before a jury, then that party must make a formal request to the court in their initial pleading for a jury (this is typically called a “demand”). In most civil cases, usually at least one of the parties will request a jury trial, so it is quite common to have a jury in most civil cases.
WHY would neither party in this case have wanted to have a jury?
Again, the opinion does not say. It is possible that one of the attorneys may have simply forgotten to make a “jury demand” and as a result, his or her client may have lost the opportunity to have a jury decide the disputed facts in this case. (In such a situation, this might even have been negligent by the attorney who subsequently might even be sued for “malpractice” by his former clients) However, it is more probable that (at least for perhaps different tactical reasons) the attorneys for both the plaintiff and the defendant each believed that it was in the best interest of their respective clients for the trial judge to decide all of the relevant facts instead of the jury. (E.g., perhaps the defendant’s attorney believed that the trial judge would be more sympathetic to their 5-year old client than a jury might have been; or, perhaps the plaintiffs’ attorney feared that a jury might not award any significant money damages against a 5-year old boy, and decided to rely, instead, upon the trial judge’s decision).