If players insist on splitting the party, you can generate extra characters for people to play to keep them occupied when focus is on one of the sub-groups.
Of course, this leads to having multiple character groups, who will probably just split up again. In the limit, everyone in the world is a PC.
Commentary by memnarch (who has not seen the movie)
There's the stablehands. Who are street urchins now as well. Hm. Not having the transcript with the preview, I think that Sally is the one on the right in the first panel, as Pete's more likely to immediately jump to the idea of stealing.
I guess opening the stable door doesn't immediately let the fathier stampede out. Makes sense I suppose, since there's a stablehand sitting around inside, and the riding beast immediately escaping wouldn't match with that. I think that the fathier would wander out if Rose moves out of the way though. They're probably not that well trained or broken to just stay put, however, with how sparse the inside of the pen looks and the treatment during the race.
I am a little curious why movie-Rose would open the door. Maybe she happened to see the kid through the door and wanted to chat? There's not going to be any way for me to tell one way or the other from the comic of course, but it's one of those little things I'm interested in finding out much later.
Transcript
GM: Two young street urchin stablehands regard you with curious eyes.
GM: Sally, you can play Arashell Sar, and Pete can be Oniho Zaya.
Arashell Sar: Cool! We’re street urchins, you say?
Oniho Zaya: I eye these strangers for an opportunity to pick their pockets.
GM: I made these three so everyone could have a character on Canto Bight.
[SFX]: click {Rose opens the fathier stall door}
Arashell Sar: “Three”?
GM: Did I say three? Well... There were three urchins, but Jim jumped in and created DJ, so—
Temiri Blagg: {revealed sitting inside the stall} Ooh! I want to be an urchin! What’s my name?
GM: Uh... Temiri Blagg.
Temiri Blagg: ’Allo, guv’na! You ’ere to lend some poor dustbin lids an ’and?
Oniho Zaya: This is your fault. You said “street urchin”.
Temeri Blagg: Blimey! Is there some sort o’ wife?
Oniho Zaya: ... Cockney rhyming slang is not commutative!