Looking at a used Sony a6400 with the 35mm 1.8 for weekend street and kid photos around Chicago (~$900 total). If this sticks, where does this go next — am I better off starting on this path, or will I hit lens/AF limits versus a Canon R10 or Fuji X‑T30 when I move to low light and short video clips?
But > versus a Canon R10 or Fuji X‑T30 when I move to low light and short Stick with the a6400: AF/tracking for kids is ahead of the X‑T30 and on par with R10, and E‑mount has cheap upgrades (Sigma 18‑50 2.8, 56 1.4) so you won’t outgrow it at $900. The 35/1.8 OSS handles low light and quick clips, just remember there’s no IBIS — use OSS glass or a tiny gimbal if handheld video matters.
Is 4K60 or IBIS important to you? I’m with @labsummit on tracking — your $900 a6400 + “35mm 1.8” is a solid start for Chicago street and kids; the real gotcha is no IBIS — — so add a Tamron 17–70 f/2.8 VC next for steadier clips and more reach. For fast little humans, use AF-C with Face/Eye on and a minimum shutter around 1/250, and you can always jump to an A6700 later without changing systems.
If you go a6400, get the E 35mm f/1.8 OSS version and plan your next step as the Sigma 18–50 f/2.8 DC DN or the stabilized Tamron 17–70 f/2.8 for steadier handheld “video clips” and indoor family stuff (17-70mm F/2.8 Di III-A VC RXD (Model B070) | Lenses | TAMRON). Small caveat: quick pans show more rolling shutter on the a6400; if you think 4K60 later, the R10’s cropped 4K60 is the only one here that does it. Do you already have the OSS 35 or the FE 35?