Celestia and Luna were never exactly baby-crazy. They'd both experienced brief romances in their youth, and nothing ever lasted long. They'd dedicated their lives (well, Nightmare Moon incident aside) to their kingdom. There were many mares and stallions throughout the years that they'd forged strong mentor relationships with (notably Twilight, of course), knowing the pride they felt for these individuals matched that of any mother's. Cadence's dedication to her upcoming roles was unwavering, but she was more open to the world of romance. Her marriage to Shining Armor and the consequent birth of Flurry Heart delighted the royal sisters. They spent as much time as time would allow with the infant alicorn, marveling at her abilities and adoring her utter adorableness.
However, they still had many duties to attend to and trips to the Crystal Empire weren't as frequent as they'd hoped. They still were able to watch the spunky little filly grow into a lovely young princess, her mother standing as a strong, courageous, and empathetic role model. But then, Twilight Sparkle, the rest of the Elements of Harmony, and their family members began to all get married, then invitations for baby showers began to flow in. Much to their surprise joy, it was the fourth princess herself that was the first of the bunch to shyly admit she was with child. Starlight Glimmer and Fluttershy weren't far behind, and soon, they found themselves just buying presents in bulk because there were so many new arrivals.
As they slowly transition more responsibility to Cadence and Twilight, this leave Celestia and Luna more time to indulge in absolutely spoiling the little fillies and colts that came into their lives in one way or another. They loved indulging in their various little displays of magic, flight, or strength, buying treats, playing hide and seek, and so on and so forth. The little ones bring a new sort of joy into their lives that puts a refreshing spark back into them.
Cadence, despite already reveling in motherhood herself, was always eager to take up babysitting duties when time permitted. She utterly and absolutely adored her daughter, but another infant just never entered her and Shining's lives. Instead of feeling sad, they instead enjoyed visiting their gaggle of "nieces and nephews," bringing fun toys and treats from the Crystal Empire and revisiting all the great feelings that surrounded itty bitties. While they got older, the visits for some of them became less frequent. But none of the princesses have forgotten their little friends, and always stand on the sidelines with words of wisdom and encouragement when needed, watching the next generation grow.