I’d imagine at first that the kids stay mostly hidden. First it’s to make sure Koichi won’t find them and send them back (they managed to use Bohemian Rhapsody to hitch a ride in the first place), and they mostly watch from afar. Maybe they end up witnessing the Black Sabbath fight (but not the lighter extinguishing), which just affirms their mission even more because tHEIR OLDER BROTHER IS SO COOL LOOK IT THAT >:00000 HE MADE THAT SUPER COOL TREE AND FLOWERS WITH HIS STAND!!!!!!!!!
and just. The whole time they’re secretly tailing the Bucci Gang and trying to stay out of sight since When We Meet Hims Properly It’s Gotta Be Cool, and therefore the only way to impress their older brother is to help him out along the way!
and just. Occasionally the Bucci Gang would get slight glimpses of the kids. Nothing concrete, just this ominous feeling of being Watched and having no idea where it's coming from and not being able to find them. Occasionally Narancia will see the occasional blip on their radar, but they always disappear before they can find them (that's thanks to Donatello hiding them in the ground)
the train was the first time they got actual solid evidence of the kid's interference and it was...... weird. Mista really had no idea what the heck was going on, one second he was on the floor bleeding out and rapidly aging, the next he's in a weird hole with a fully stocked clinic and feeling kinda chilly with a bunch of tiny hands touching him. The others were skeptical to believe him, but he has the bandages to prove it so ?????
and maybe when they decide to defect is when the kids decide to make their grand appearance. They used Bohemian Rhapsody to send a message of "hey, we can help each other, following the swirling rods if you wanna met up" and lead them all to a weird Not At All Suspicious abandoned building
The Bucci Gang were not at all prepared in the slightest to deal with these Superpowered Children
they were even less prepared when the four of them introduced themselves as Giorno's siblings and show their star marks to prove it, Giorno being the most confused out off all of them
they end up make the mistake of assuming that Jolyne was the most harmless since she didn't have a Stand, but she's the most unhinged and ready to throw down at a second’s notice. One Does Not Simply Have Three Stand User Basically-Siblings And A Stand User Father And Other Stand User Relatives Without Picking Up Some Tricks
also speaking of, you bet Jotaro is loosing his fucking mind right now with worry over where the kids are
also also, completely separate from that, I had an absolutely ridiculous idea
instead of Giorno getting a Requiem Stand....... what if it's Jolyne, just for the shits and giggles of it