You cannot say that it's entirely true that kids learn a second language easily, but it's still the best time to learn. Babies are basically equipped to hear and distinguish every possible language and dialect, but once they reach 6 months old, they begin to focus on the particular sounds that exist in the languages that they're exposed to. In this way, kids will naturally and automatically be bilingual  if they hear multiple languages on a daily basis when they are still babies. One lady told me of her son who grew up in a very multi-cultural community working at a resort. He would naturally and easily switch from Dutch, to French, to Italian, German and English depending on who he was talking to. Of course that's an extreme case, but it's completely possible and easy. It's not true that they will get mixed up by learning multiple languages. They may mix them up purposely simply because certain words or expressions are easier, but they won't develop any permanent language disabilities. Quite the contrary, bilingual children have been shown to be as much as a year more advanced in learning ability development for 2 and 3 years olds. Some kids who simply aren't gifted at learning languages, just as some aren't gifted at sports or music, will find it difficult to continue schooling in a foreign language. In that case, it may be better not to continue with the foreign language school. Perhaps for Grade Two you could try a bilingual school. In any case, learning even a small amount of a foreign language, maintaining the ability to hear and pronounce it by listening to music and movies will keep a foundation for later learning. Knowing a second language can be a great benefit for gaining employment. What should you do if your kids are not good at learning language?
Send them to a bilingual school later. Ask them to turn to sports or music. Encourage them to remain in Grade One. Let them remain in the foreign language school.