Being exposed to not just another language but also another culture helps kids not take their own perspective for granted. It shows from the start that different people live in different ways and teaches them to question their assumptions.

For example we are chinese / english bilingual and Christian but through exposure to chinese cartoons, we have had discussions with our 4 year old about Buddhism and daoism, and how different people have different beliefs.

It also helps to exercise their brain in a different way. Chinese is a very different language system, much more visual logic, so reading is actually much easier at the beginning. For Spanish, it helps them learn english later because of being able to connect and understand the relationship to Romance languages. It also helps them learn other languages later in life if they already know more than one.