FireOS does not include access to the Play Store, but adding it is really just a matter of installing three APKs and signing in to your Google account if you feel like you really need it. The Amazon App store does have some pretty big holes in it; Firefox isn't available in it, for example.
Amazon devices have nice, bright screens and good battery life in their favor. They also have a fairly straightforward launcher. I'd also put forth that Amazon FreeTime is a pretty good sandbox kid mode, but Amazon did finally make it available outside its own devices as of a few weeks ago. No, these things aren't $400 super-premium tablets. But for the money, it's hard to complain about what Amazon is selling. These guys are worlds nicer than $50 No-name Chinese knock off devices.
Down side is that they're hard to root. The default launcher doesn't support widgets (not that I miss them) and has a somewhat limited subset of configurable options compared to Android + the Play Framework. But I've bought these for my nieces and since they do Youtube and Minecraft for cheap, all is well.