I lived in UC from 1980-96. I moved out during college (CSUH represent!) so I'll give you what it was like to grow up there so you can decide if it's a good place to raise your kids.
The Union in UC comes from the communities of Decoto and Alvarado coming together in the mid 1900's to better advocate for themselves. It was a town of gladiola fields and if you look up the recent history of California you'll get some context about that. After WW2 silicon valley blew up (look up secret military history of silicon valley on youtube). So you have 2 agrarian communities coming together at the same time the fields are getting paved over for burbs.
That led to a suburban city with a farm laborer community and a middle class community. They got split by 17 which became 880 (though old alvarado was still poor out by the old dump).
So, I grew up in the UC k12 and I actually got a pretty badass public education. The reason for all the backstory is it explains the gangs. The farms got crowded out for development. The strip mall with the walmart and such used to be a gladiola field and drive-in theater. The jobs went away. Walmart isn't a job-it's just something you do on welfare so you don't go nuts from boredom. And desperate people do desperate things. So gangs. Yeah, they were around before the mall, but the reasons behind it got more acute with the surge of development in the 90's.
So yeah, there's a split between poor people of color on one side of town and middle class somewhat mixed on the other. The elementary schools and middle schools are geographically split but feed into 1 high school. Your kids are going to have the benefit of a great school system but will eventually have to encounter and negotiate every gangster in town.
Let me illustrate that. In my initial Marine Corps training I ran into a gangster from Oakland who joined up for a better life. We shot the shit for a bit and when I mentioned UC he froze, got a 1000 yard stare, and said "yeah... I been to UC". Don't get me wrong, it's not the ghetto, but shit gets real in UC.
In some ways Union City is just like any burb. My advice to parents though is have an exit strategy for your kids. Mine set us up for state college (good luck, CSU was a lot cheaper then) and gtfo'd once we moved out. I suggest you do something similar if you can wing it.
Also, facebook corporate are open society zealots. Move on as soon as you can. It doesn't help your kids to escape UC if you build a society with no escape.