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.
In any case, we're guessing the writers are giving Zola a bigger role in the series — perhaps even a few lines of dialogue. ("Hey, Daddy, are you really working for the president? What about your promise to let your wife advance her career while you cool your jets a little?")
And hey, congrats to Heaven to landing such a fun part! We certainly wouldn't mind being fawned over by Patrick and Ellen.