Everyone says the petitions are processed separately and will come through at different times, and to wait at NVC stage until all are gathered as approved. BUT, from everything I've read of everyone's experiences, if you've correctly mentioned the others in each I-130 where it asks if they're immigrating together, they are typically approved simultaneously. One recent case on here of a June filer saw that her spouse got an RFE and the same day both kids were approved.
I do wonder about the different service centers though. We are also petitioning for 3 - spouse and two kids - and the spouse and one child is at Nebraska (transferred from Texas), and the other child is in California (transferred from Texas).
Lastly, I find California and Vermont processing times on VJ have been out of whack for a few months. They are fluctuating significantly. They're all approving mid-June right now.