They do not have to curtsy/bow to the new reigning monarch, and Queen Mary did not have to curtsy to her granddaughter or sons. All "His/Her Majesty"s are on equal footing and don't curtsy/bow to one another, whether they are consorts or regnants. However, Queen Mary chose to curtsy to the new monarchs the first time she met them after their ascension in order to impress upon them the significance of their new role and the difference in who they now were. I don't believe she did it more than once for any of them.
There is a funny story about Queen Mary's being received by the new QE2: after curtsying to the new young Queen, she solemnly said, "God Save the Queen." Then she continued, "Lilibet, your skirt is much too short for mourning." I guess Granny is always Granny...