importance of fluency at early learning

I thought about putting this in the home school forum but really this is about teaching my children a second language. I really want to home school to continue the acceleration of my children’s learning, but there is a public dual language school in the area. Half the classes are in Spanish and the other half are in English. They also balance the students as best as they can with half being from a Spanish speaking home and the other half from an English speaking home. I am keen on my children speaking a second language, but I am unsure if I can provide them an experience of speaking fluently as a dual language school will do for my children.

I know that I can “after school” the children, but I am unsure if their acceleration will be a problem in the classroom. Has anyone with older children really had success in fluency without being in a dual language environment?

If fluency is not gained at an early age (0-10), will it be too difficult to gain fluency when they are older and have the opportunity to travel at their high school or college level? Thanks for any opinions or success stories or even your not very successful stories. I am just trying to gain perspective on how best to give the gift of a second language to my children.