I got the S5 when I was pregnant with Connor because it had the best camera available at the time, and I knew I'd be relying on my phone to capture all the super cute baby moments. I didn't even think about it being shatter and water resistant. As a parent, especially with a newborn, you really need shatter and water resistance. I can't tell you how many times Connor spit up on that phone, or dropped while I was hold him, or (because you should never debone a turkey while tired) dropped it in the a bunch of turkey juice... That thing is still going strong. Plus, Kids Mode on the S5 is arguably the BEST THING EVER. It locks out access to anything but the apps you approve. He can only make phone calls if I set that section up for him, and it has this adorable little gator that reacts when you touch him.
The S7 is more durable and claims to be waterproof (I haven't been brave enough to test that yet), but Kids Mode is a joke. In the S5, it's a completely separate version of your phone. In the S7, it just adds a few restrictions, but they still have access to most everything on your phone. Now, I use my S7, and Connor got to keep my S5. It doesn't have service, so he can only access things like Netflix and Disney Movies on the go when it's attached to WiFi, but it still has all of his PlaySkool apps and that awesome gator.