Ever have a briefcase or combination lock where you can change the combination via a procedure? And once you do, the new combination IS the combination, and the old one doesn't work anymore. That's all there is to a Kwikset so-called "smart lock". There are no smarts. So once the new key-cut is learned, the old key-cut no longer works.
Keys have a "cut". It's the jagged side of them. If you look closely at different keys, they have different cuts. Kwikset uses 5 cuts with 7 possible depths of cut. You can look at two keys and see whether they have the same cut or not.
Remember the Kwikset locks can only be set to work with one cut. So everyone's key needs to have the same cut.
Inventory all the keys you have, sort them by cut so each group of keys has the same cut. Once you find a cut that you have enough keys for, re-key your locks to use it.
The Kwikset "smart lock" does not manufacture cut keys. That requires metal-working, although it's an easy job for anyone with a key-cutting machine. A locksmith, or a hardware store, a drugstore, or even some automated kiosks seen at big-box stores. They typically take a key you already have and make a copy of it. Though at a better shop if you said "Surprise me!" they could make you a random cut of key.