Clara had thought it through carefully before deciding on the dosage. 9 drops to take herself back to 10 and 45 drops to take her grandmother back to 50. That should avoid any big changes to the familial relationship with the reality adjustments. More importantly it would leave her old enough to gather the reversal ingredients without much difficulty when the time came.
Giddy as she felt the magic begin to flow through her, Clara imagined what fun things she was going to do on her youthcation. Of course there would still be a couple months of 4th grade left before school was over which was kind of a bummer, but wouldn't be nearly as rough as the finals she was dodging.
But after that she was free to enjoy the Summer. Which meant all the freedom of a soon to be elementary school "senior". Which meant bike rides, sleepovers, and other things she's been too old for for a while. Plus with her grandmother younger they'd probably have more exciting plans than Clara got to experience the first time around.
By this point the magic had begun the process of rewriting the world to accommodate the more youthful duo that was about to emerge. Inside the house furniture and decorations were shifting and changing, government databases were being rewritten to accomdate the new birth dates, neighbors and friends were having their memories altered, and more.
Even in her own mind Clara could feel things beginning to reorient themselves. She didn't panic as this was explained in the potion book. In order to avoid difficulties adjusting to her new age and reality, some parts of her mind had to be altered.
After all it would be hard to adjust if her mind still thought her arms and legs were longer than they were. Or if she had the food, drink, and entertainment preferences of an 18 year old. Or if she didn't speak like a current 10 year old. Or most importantly if she didn't have the day to day knowledge expected of her; teachers, friends, homework, and the like.
For the uninitiated changes to the mind like that could be scary, but Clara understood magic. Well mostly understood magic. In either case she was prepared no matter what changes occurred. She held her eyes shut for a moment as the magic intensified before suddenly fading. Opening her eyes Clara prepared to examine her handiwork.