<data>
    <data>
        <username>johny1994</username>
        <realname>John Batcher</realname>
    </data>
    <data>
        <username>drdre222</username>
        <realname>Doctor Dree</realname>
    </data>
</data>

tinyxml2::XMLDocument doc;
tinyxml2::XMLError e = doc.LoadFile("text.xml");
if (e != tinyxml2::XML_SUCCESS) {
    std::cout << "Error load file" << std::endl;
}

auto root = doc.FirstChildElement("data");
if (root == nullptr)
{
    std::cout << "Child element not foundn";
}

// Тут должен быть какой-то код типа цикла для обработка data элементов
// что-то типа for (root; root < elementCount; ++root) {};
auto rootdata = root->FirstChildElement("data");
if (rootdata == nullptr)
{
    std::cout << "Child (data) element not foundn" << std::endl;
}

auto username = rootdata->FirstChildElement("username");
if (username == nullptr)
{
    std::cout << "username is null ptr" << std::endl;
}
std::cout << "Username:t" << username->GetText() << std::endl;

auto realname = rootdata->FirstChildElement("username");
if (realname == nullptr)
{
    std::cout << "realname is null ptr" << std::endl;
}
std::cout << "Real name:t" << realname->GetText() << std::endl;

Username: johny1994
Real name: John Batcher
Username: drdre222
Real name: Doctor Dre