Take a look at the original sentence again:
Sherian refuses to let her kids have a puppy, she
is convinced that dogs are just big germs and refuses to follow one around
with a sponge and bottle of bleach.
You wanted to fix it this way:
Sherian refuses to let her kids have a puppy, but
she is convinced that dogs are just big germs and refuses to follow
one around with a sponge and bottle of bleach.
Using a comma and a coordinating conjunction like but is a perfectly good way to join two main clauses. You must be careful, though. But shows contrast. In the example above, no contrast exists, making but an illogical choice.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.