Apple Watch 3 won't pair with iPhone 6

I bought my daughter with autism an Apple Watch 3 to pair with my iphone 6 so that she can function at school. Now that I have the watch I find that Apple no longer updates iPhone 6's iOS just because. No explanation. So now the watch is useless, my daughter is heartbroken and I am furious. They want people to buy yet another **** phone from when they are already enormously expensive. It's greedy and unnecessary. I should have sorted her out entirely with Android.