That's an interesting take!! :0
Originally, for this idea, I was going to have Grace's mind de-age along with his body because I wanted it to serve as a character study of sorts:
To Rocky, the only Grace he knows is... well, Grace: a former teacher at Grover Cleveland Middle School, who has a PhD in Molecular Biology. He’s tall, brave, brilliant, and, of course, the savior of the stars.
But he doesn't know Ryland. The boy who, while open-minded and eager to learn about everything and anything, is rather small for his age, reserved, afraid of the dark, and has trouble making friends.
So, it'll be interesting to see how Rocky tries to regain Ryland's trust. Comforting a little boy who's light-years away from home while simultaneously racing to develop an antidote.
But! If Grace were to keep his adult mind intact-OH! how it would STING.
Sure, it can be played for laughs with how ridiculous his voice sounds now and how he can't reach practically anything, but the absolute TOLL it'd take on his brain and emotional regulation.
A child's brain simply isn't built to hold over forty years' worth of memories, experiences, and knowledge. It's like trying to pour more water into a glass that's already overflowing. Eventually, it stops fitting, and everything starts spilling over.
His memories are going to become more scrambled and jumbled than anything. He's going to get mad at himself for not remembering simple equations. He's going to find himself throwing tantrums and crying more regularly over things he'd normally brush off.
And, most importantly, he's going to crave the comfort of his mom, knowing damn well she's gone, and yet his brain refuses to believe it..