"Because if something does go wrong, I'll be there if you're hurt or unconscious or if it can be fixed but only so quickly, or... or... whatever."
"And when you have been casting spells for a decade, I will no longer be supervising."
The next day, as a wordless not-exactly-apology, she shows Isabella a list of spells she thinks she should work towards. It's not their full curriculum by any means, but the last spell listed is a magical lock for her diary.
In case Isabella was interested.
Isabella seems to have decided that for now her strategy is to make insisting on magic supervision extremely time consuming.
Classes other than magic will still occur. (Vivian leans heavily on published lesson plans; she's home schooling Isabella for reasons like 'she can add magic lessons' and 'she can give a less Western-focused view of history', not 'she is qualified to teach in any meaningful way'.) She continues to take Isabella to rallies and write angry letters to relevant politicians and create charms to sell in Avalon. None of these are particularly conducive to casting light spells.
On the other hand, Isabella is more than welcome to learn to make charms of her own. Is that a project she would be interested in?
Then, when next she's heading to the Bay Avalon to sell her latest batch of charms, she invites Isabella along. It's her first trip since Isabella got her medallion, and Isabella helped make some of the charms, after all.
Isabella is very proud of this fact and will tell anyone who seems like they might listen. She is walking on her hind paws because she has never learned to get quite used to shoes (she will wear flipflops if she has to be all human, grudgingly; this is not a problem with Bay Area weather) and she has her wings and tail, but no other sphinxy parts.
It's a large Avalon, and a busy one, so the ferry line is crowded; the ferryman recognizes Vivian and nods her through, with a quick smile for the cute small child trailing her, but then promptly returns to his work as bouncer. There are plenty of people milling around on the deck, enough of whom know Vivian that Isabella will have plenty of people around to tell about her charms if she so desires.
Only one person is stupid enough to then object (out loud) to the idea of a young girl doing magic; Vivian drags him off to lecture him about daring to accuse her of poor parenting, in front of her daughter no less, when all her magic is carefully supervised and double checked and of the tiny and harmless variety. He does not appear to actually change his mind, but he is successfully intimidated into apologizing to both her and Isabella, which Vivian deems barely adequate.
She does not mention Isabella's early morning light spell adventure. It is none of his business.
And then, after some time puttering along the water, the ferry arrives at their destination: the Bay Avalon. The name is quite literal; the Avalon is in fact floating on the Bay, hidden by some impressive concealment spells that Vivian can only dream of learning.
"Welcome to Avalon, love," Vivian says with a smile. "Where to first?"
Isabella has never been here before, so she scarcely knows; "everywhere" does not seem like the right sort of answer. Her tail lashes. "Can I fly, here?"
"Don't go above the church spire, all right? They don't really have a way to mark how high the spell goes. So if you stay low and where I can see you, you can fly."
Isabella identifies the church spire - then nods, and takes off. Whee! What is there to see here?
Non-water creatures such as tiny sphinxes are not recommended to participate in the swimming. The bay is cold.
Isabella does not attempt to swim. She inspects the lay of the land, then espies something that looks like a library and lands on it and from there glides to the ground in front of the entrance.
Vivian meets her in front. "Would you like to stay in the library for a bit?" she asks. "I can come with you if you want."
(Also, she whispers to her mommy, "Can I get sphinx books?")
Bella goes from stack to stack and gets many many things.
"Ready to go sell some charms, love?"