There is no 'quick fix' solution to acquiring a new language, though advertisements for some products on the market might tell you otherwise. No child can become a bilingual marvel overnight. What is true is that young children have an amazing, natural ability to learn languages.
I have two children, one approaching three years, and the other having just turned five. I have watched with fascination at how, in the two years we have been living in Spain, they have soaked up Spanish. With a local accent of course! That's unavoidable - kids copy.
I also know a family with two boys of a similar age. Like us, they have been living in Spain for the last two years. Their Danish mum speaks to them in Danish at home, and their English dad speaks to them in English. Meanwhile, they go to a Spanish school. Now, here's the important point. The boys have roughly equal EXPOSURE to the three languages, which is why they speak all three of them well. Similarly, my two children have roughly equal exposure to Spanish and English. We speak English at home, while at school and on the street it's Spanish all the way unless they are mixing with some of their English speaking friends. The consequence of all this is that their Spanish is as good as their English. Remember, they have only been here for two years.
The more your child hears a foreign language, the more they will absorb it. The more opportunities they get to speak it, the better they will be able to speak it.
Young children have brains capable of learning different languages in parallel, at the same time, so when choosing a foreign language course for your child, aim for something with as much exposure as possible to that foreign language. You needn't worry about them forgetting their English. That simply won't happen.