Family law trials are "bench trials" in Washington. That means that the judge determines the facts as well as the law. In most other kinds of cases, the judge still determines the law, but the jury finds the facts. ("Finds the facts," means that in a he said/she said dispute, the "fact finder" determines whose story, or what mix of each story, becomes "legally true," for that case, and the law is applied to those "legal facts.")
How does this all matter for family law? The appellate judges prefer "finality" in family law cases so that you can all move on with your lives. Therefore, the appellate courts will not substitute their judgment for the trial judge. Instead, your appeal can only be won: (a) if the judge made an error of law (the appellate court does not "defer" to an error of law), or (b) if the decision lacked "substantial evidence" (meaning that there was so little evidence in support of the judge's determination that no reasonable person could have found those facts -- here the appellate court defers to the trial court), or (c) if the result was overall manifestly unreasonable.
As you can see, on the facts and on the judgment of overall fairness, the trial court has a lot of leeway and the court of appeals will not re-examine any reasonable decision. However, if there were legal errors, they will be reviewed carefully, and the matter will be sent back to the trial court for more fact-finding and more legal argument.