My kids have grown up in a bilingual home and they have always known "Daddy's language" and "Mommy's language" and are able to speak to both of us in our original language. Yes, there are times when they will mix words or they will know how to say something in one language but not the other, but these are great times of learning for them.
Most of us don't speak two languages in the home, but you can still easily teach your child another language. Even if you don't know it yourself, there are many YouTube or free online resources in your target language that are fun for kids.
One of the best things with young kids is to present them with easy, everyday vocabulary. Practice with objects around your house and make a game out of it. I love using the "Simon Says" game to help teach action verbs.
Another great way to keep your kids attention while learning is to play the normal games that you usually play with them but introduce a language component. For example, our kids love to play Uno but practice saying the numbers and colors of each card in another language. This repetitive type of game-play reinforces what the kids are learning.
When learning a foreign language, the hardest part is learning how to speak it. Learning the language at home gives your kids a non-stress way to practice saying the words and eventually speak easy sentences. With young kids, you may not want to cover grammar but have them focus on learning key phrases instead. The grammar will come along the way, instinctively like it did with their first language.
Learning a new language with your child also opens the possibility of learning about the cultures that speak that language. Learning history or a foreign cuisine is even more interesting since the kids are learning the new language at the same time.
For those of you that hire a nanny or babysitter to watch your children, you may want to consider one of the nannies at the website ForeignLanguageSitter that speaks your target language. These nannies specialize in helping your child learn and speak their native language.
So why not try teaching your children another language? Who knows, maybe while your kids are learning, you might actually learn too!
