As a first approach, I'd collect a number of measurements from each of as many knowns as you can find or make, and plot them for a visual clue to the linearity or not (probably not) of the sensor's output vs. pH. You could try to eyeball-fit a curve to the data; interpolate linearly between sample points if you have enough of them; or run a least-squares approximation fit. The key is you'll need enough data - knowns - to make whatever approach you use give results accurate enough for your intended use. (Obviously this will be different if you just need to know if something is acidic or basic, or you're monitoring tropical fish tanks, etc.)
Update: If your pH meter is good enough for the fish, and if your sensor is reliably repeatable and sensitive enough, why not check your meter against the two standards, then calibrate the sensor to your meter? Once you're happy with the result, lock down the screws with a dab of nail polish (it's not very tamper-proof but it will be tamper-evident). A pH 10.0 standard won't be very useful to you; at that extreme you'll already know the pH is out and you probably won't care about the exact value. You need your precision between the fish-tolerable limits, close to the pH goal. In fact, if you trust your meter, you don't even need the standards. In that case you need only make up a few samples as you described, covering the pH range of interest, and use them to calibrate your sensor to the meter.