I've seen many aus where CaitVi meet as kids and start out pinning for each other since then, and they are hella cute, but personally (and this might be a hot take) when they are both kids is when they least have chances to actually fall for each other.

Part of the reason they both fell in love in canon was essentially because they went to war with just one another for a week. They went through a trail by fire that revealed who they truly were inside AND forced them to trust the other, the kind of absolute honesty which lingers and causes people to fall in love.

A younger, less rebellious Cait because she doesn't have as many years of her parents (mother) being overbearing might not have enough fire in her to consistently stick to Vi like she did in Arcane. Similarly, a Vi that has other priorities like protecting her entire family might not be as quick or as open to trust a piltie who hasn't proven herself.

If it happens I think it'd have to be a much slower burn over long periods of contact (assuming it deviates from what happened in Arcane), and if it happens Caitlyn and Powder having a nice friendship would be the catalyst, something that ironically I could see happen much easier because Powder needs the approval that Caitlyn would give when she gushes over her inventions.