Here is the sentence again:
When Simone and her husband discovered sixteen jars full of insects in their
daughter's room, they decided to get Maria a dog,
tolerating one four-legged animal seemed
better than having fifty six-legged "pets" in the house.
They decided to get Maria a dog is a main clause. Tolerating one four-legged animal seemed better than having fifty six-legged "pets" in the house is a second main clause. Connecting these two main clauses with a comma makes a comma splice, a major sentence error.
A period after dog and a capital T for tolerating would fix the problem.