You can do it with javascript easily. Create and make new array according to your old array.

var oldArray = [
    {
        "parentId": "uniqueParentId1",
        "parentName": "Parent1",
        "childProp1": "test1",
        "childProp2": "test3"
    },
    {
        "parentId": "uniqueParentId2",
        "parentName": "Parent2",
        "childProp1": "somevals",
        "childProp2": "other vals"
    },
    {
        "parentId": "uniqueParentId2",
        "parentName": "Parent2",
        "childProp1": "somevals 1",
        "childProp2": "other vals 1"
    }
];

var newArray = [];

for (var i = 0; i < oldArray.length; i++) {
    var currentObject = newArray.filter(function (x) { return x.id == 
oldArray[i].parentId })[0];
    if (currentObject == null) {
        var newObj = {
            id: oldArray[i].parentId,
            name: oldArray[i].parentName,
            children: [
                {
                    childProp1: oldArray[i].childProp1,
                    childProp2: oldArray[i].childProp2
                }
            ]
        };
        newArray.push(newObj)
    }
    else {
        currentObject.children.push({
            childProp1: oldArray[i].childProp1,
            childProp2: oldArray[i].childProp2
        });
    }
}
console.log(newArray); // your array is here

You can do it like this: loop through your array and create a new array with the data you need, i didn't know what the name variable was supposed to be so i generated it.. if that is supposed to be the parentId number you can extract the digits from o.parentId I've also assumed that except parentId all other object keys are childs...

let arr = [{
    "parentId": "uniqueParentId1",
    "childProp1": "test1",
    "childProp2": "test3"
  },
  {
    "parentId": "uniqueParentId2",
    "childProp1": "somevals",
    "childProp2": "other vals"
  },
  {
    "parentId": "uniqueParentId2",
    "childProp1": "somevals 1",
    "childProp2": "other vals 1"
  }
];


let combined = {};
let parentid = 1;
let childs = {};
arr.map(o => {
  if (!combined[o.parentId]) {
    combined[o.parentId] = {
      id: o.parentId,
      name: "Parent" + parentid,
      children: []
    };
    parentid++;
  }
  childs = {}
  for (const [key, value] of Object.entries(o)) {
    if (key !== 'parentId') childs[key] = value;
  }
  combined[o.parentId].children.push(childs);
});
console.log(combined)

You can do:

const flatObjects = [{parentId: 'uniqueParentId1',parentName: 'Parent1',childProp1: 'test1',childProp2: 'test3',},{parentId: 'uniqueParentId2',parentName: 'Parent2',childProp1: 'somevals',childProp2: 'other vals',},{parentId: 'uniqueParentId2',parentName: 'Parent2',childProp1: 'somevals 1',childProp2: 'other vals 1'}]

const result = Object.values(
  flatObjects.reduce((a, c) => {
    const k = `${c.parentId}|${c.parentName}`
    a[k] = a[k] || {
      id: c.parentId,
      name: c.parentName,
      children: [],
    }
    a[k].children.push({
      childProp1: c.childProp1,
      childProp2: c.childProp2,
    })
    return a
  }, {})
)

console.log(result)