I feel like baby Zoro would get really angry at baby Sanji at first because he's such a crybaby who won't stand up for himself, but after realizing this kid gets abused all the time he'd go feral over anyone who even looks at him wrong.
So now baby Sanji just follows baby Zoro around everywhere like a puppy and Zoro is like totally fine with it.
Kuina makes fun of him all the time because why is Zoro so invested in this random kid? Zoro doesn't see what's so strange about it and honestly he kinda loves how Sanji cheers on Zoro during his spars with Kuina. Even though he loses every single time Sanji still tells Zoro he's so so cool.
At some point Zoro would tell Sanji to just not go back home and Sanji is now a runaway that Zoro hides in his room. The master finds out eventually, but lets Sanji stay at the dojo even though he's not a student. Sanji helps in the kitchen and is really good at chores so it's a fair trade.
Even though they are definitely a lot nicer to each other in this timeline they are still them so once they get older they start bickering all the time. Sanji starts wanting to "be his own man" at the ripe old age of 13 and starts pushing back and getting angry at Zoro for always fighting for him. Zoro is angry at this and low-key hurt that Sanji doesn't want his protection anymore since that's the only way Zoro knows how to emote.
It all comes to a head one day when the arguing crosses a line and fists (Zoro refuses to use an actual sword against Sanji which makes Sanji even more upset) are thrown. Zoro obviously wins at this point because Sanji has no formal training and it ends with Sanji angry crying.
Sanji basically says he can't stand being in Zoro's shadow anymore and he wants to get far far away from him. Zoro is triggered because Kuina is dead at this point and now the only friend he has left wants to leave too? So Zoro tells Sanji in the coldest voice he's ever used that Sanji should leave then. If he's not around slowing him down and distracting him anymore then he'll get strong enough to achieve his goal in no time.
They both make some dramatic declaration to sever ties and be rivals from now on. They'll each get stronger in their own right and one day when they are gown there will be a battle to determine who is better once and for all.
Sanji packs his things and leaves the village. Zoro refuses to look at him while he walks away. Sanji doesn't look back either.
From then Sanji & Zoro's story go back to the original canon. Zoro trains non stop until he leaves and Sanji still joins a ship to learn cooking and gets stranded on that rock with Zeff. Really the biggest difference is they each have each other in the back of their minds all the time in order to push themselves harder.
When the crew meets Sanji at the Baratie him & Zoro instantly recognize each other, but won't say anything about it to anyone else. Zoro spends the entire time trying to get Luffy to give up on recruiting Sanji. Sanji is even more resistant to joining. However you can only resist destiny so much and Sanji still ends up joining the strawhats.
They never tell anyone about their history and everyone thinks they just have incompatible personalities. If you pay attention though you'll see that they know a bit too much about each other for having just met a couple years ago. The insults seem a little too personal right from the get go.
Zoro will sometimes instinctively shield Sanji during a battle which sets Sanji off. Sanji somehow knows where to look for Zoro when he disappears. Almost like he's had a lot of experience with finding a lost Zoro and can guess which way he would be inclined to go.
When they are both drunk you can find them alone and laughing like hyenas. Whispering and reminiscing about childhood hijinks. If you're lucky you might overhear Sanji say "you're still so cool. it really pisses me off mosshead" and a "Guess you were right about needing me gone to get stronger. I hate thst you were right..." from Zoro.
If you're really lucky you'll overhear "I'm sorry", "I missed you" and "I'm glad we ended up back together."