Both are good.
1. You (yōu) means long-lasting and long-term. It can express your best wishes for your children, hoping for a long and healthy life and a successful career. You also means calm and tranquil, and can express a peaceful and stable temperament.
2. You (yōu) means melodious and leisurely, which means freedom and ease. It can express free, cheerful and open-minded expectations for children. In addition, "Yu" also means longing and yearning, which can express the longing for relatives and friends far away.