How to teach my kids a second language

I have three kids. Ages 4,2 and 6 months old. I was teaching my 4 years old Spanish at the beginning by only speaking Spanish to her but she was begging with speech so I felt bad and started talking in English and tv shows went from Spanish to English. Now she is way better at speech and only knows few words in Spanish. Everytime I try to talk in Spanish she says : what ? And that reminds me I have failed miserably. With my second one he only knows English because he got all in English because me trying to help my 4 years old. How can I teach my kids now the language ?