I have small hands and particularly small thumbs (the tip of my thumb doesn’t even get to the very bottom fold of my index finger. And yes, this is something that is more of a challenge when you have very small thumbs, but also...it’s absolutely doable. Sometimes I’d only get the very tip of the register key, but it was enough to clear the pad and give me a good sound. I can see how this would be a concern if you’re learning, but honestly...practice it. I second that you use the knuckle sometimes. Or you might have to slightly change the angle of the thumb to cover the hole and get the register up. My way might not work for you and others may have found different solutions that work better for them.

(But to be honest, if you keep your thumb on the hole and cock your wrist a tiny bit, you’ll be able to use the knuckle and I agree that’s the most likely solution.)