Here's how it breaks down:
* Excessive: Means too much or too frequent.
* Indulgence: The act of allowing oneself to enjoy something pleasurable.
* Spoil: To have a negative impact on a person, often by making them used to having everything their own way.
So, the sentence is saying that if you constantly give in to a child's every whim, you risk ruining their character by making them entitled and demanding.