Only there's nothing "baby" about Heaven! As you can see by her headshot here, she's clearly a much older child than the lil' actresses we've seen play Zola before.
It's not unreasonable that Zola would be older, considering she first appeared in the Grey's chronology at the end of Season 7.
But even as recently as her last appearance in Season 10, Episode 7 — "Thriller" — Zola still didn't look this old. Maybe she just had a growth spurt in the span of 10 episodes? Or maybe the Grey's producers are counting on our memory to be a little foggy.