That was a possibility.
I saw Franklin leave the ship first last month and jump about on the planet.
He didn't toddle.
Dictionary definition of a toddler is someone learning to walk, and most children can walk before they can talk.
Clearly at most a 10 year old but still older than Valeria who is a super intelligent 3 or 4 who is also not a toddler, and they were both talking in concise unbroken English.
Besides they have Alex Power walking around back home, which severely dates Franklin from his Power Pack days even if they didn't reset him exactly after Fantastic Force.
I'd blame the artist more so that the writer if anyone for your confusion. Mark Bagely doesn't know how to draw, or doesn't want to draw fat people. At his worst his characters all look like they have eating disorders.
Just reread your post and glommed onto a new word: "preschool".
Franklin hasn't gone to a real school or mingled with other children so he's probably emotionally arrested to that point in age like any hothouse flower.