JSFiddle - React, Tailwind, and code Playground

JavaScript

var family = {
    "marco": {
        "gender": "men",
        "age": 29,
        "children": {
            "jen": {
                "gender": "women",
                "age": 14
            },
            "marco jr.": {
                "gender": "men",
                "age": 16,
                "children": {
                    "Koos": {
                        "gender": "men",
                        "age": 1
                    }
                }
            }
        }
    },
    "lisa": {
        "gender": "women",
        "age": 20
    },
    "mike": {
        "gender": "men",
        "age": 32,
        "children": {
            "carl": {
                "gender": "women",
                "age": 19
            }
        }
    }
};

function getDescendents(family, memberName) {
    function inner(children, r) {
        for (var k in children) {
            if (children.hasOwnProperty(k)) {
                r[k] = {
                    age: children[k].age,
                    gender: children[k].gender
                };
                inner(children[k].children, r);
            }
        }
    }
    
    var r = {};
    inner(family[memberName].children, r);
    return r;
}
console.log(getDescendents(family, 'marco'));