<div>
    <button>
        <span><!-- using a div also works but is not valid HTML -->
            <span>Test</span>
            <span>Test</span>
        </span>
    </button>
    <p>
        <span>Test</span>
        <span>Test</span>
    </p>
</div>

button > span, p {
    display: flex;
    flex-direction: row;
    justify-content: center;
}

button::before,
button::after {
    content: '';
    flex: 1 0 auto;
}