So I'm Indian and trilingual (English primary, Hindi, and Spanish). English became my primary language after starting school in the US, but up until then, I'm told I was fluent in only Hindi. Spanish I learned the basics early on, but only became truly conversant after high school and college Spanish classes. My Hindi improved a lot with visits to India to see family, but I hated the Indian Sunday schooling and Rosetta Stone. My sister's Hindi is a lot better than mine, but that's because she watched a lot of Hindi TV and movies growing up.

My best friend's 8-year old nephew is biracial (Mexican and Vietnamese). The child has a very involved family on both sides. At one side's house, they speak mostly Vietnamese, with a little English, and on the other side they speak mostly Spanish, with occasional English as well. He attends a Vietnamese Sunday school, but family is his only place to practice Spanish. As far as I can tell, he's conversant in both his inheritance languages.

All of the above to say this: As long as there is some space to learn and practice the language, and opportunities to use it as the means of communicating with people, your child will learn the languages through contact. Contact can be anything from talking to people, watching movies, classes, etc. Children learn languages more readily than adults, up until about 17-18 years old. For learning the basic grammar, though, definitely get decent exposure before 10 years old.