Since they haven't completely locked in to that sound system, they are able to accommodate another one. It doesn't fit with math, science, and history. It fits with sitting, crawling, standing, walking, running. Adults tend to make language learning an academic pursuit. To them, it is no longer a naturally developed human activity.
Little children learn language along with everything else they are learning.
That is how they will learn a new one. Language is social and interactive when real needs are being met, when necessary messages are getting across, when actual communication is taking place. Adults try to learn language isolated from people, or they are often placed in contrived rather than real communication settings. Little children never learn words or sentences apart from people, or apart from what's going one. It is always in context and always real.