I taught English as a Second Language to children for many years…decades, in fact. Children’s brains are programmed to learn language. Begin exposing them to a second language as early as possible in childhood, during those years when language learning happens automatically.
The best way to teach a language to children is by singing songs, chanting chants, reading predictable books, building vocabulary, and repetition. It’s said we must hear something 100 times before it’s completely programmed—I haven’t seen the research to support that, but my teaching experience certainly does.
Another key feature, and it’s a major one, is to ensure that the songs, chants, predictable books, and vocabulary development all about the same topic—that way, you actually reach 100 repetitions quickly.
Predictable books are books that repeat the same structures over and over, substituting a new word or two into the sentence. This develops sentence structure and vocabulary without directly teaching it in an analytical way beyond the children’s capabilities. They will process the underlying language structures automatically because their brains are programmed to do exactly that—to seek out the patterns in the language they are exposed to.
I could go on and on, but I hope I’ve given you some idea of how to do this. Most of all, make learning English fun! If it’s tedious and beyond their capabilities, they won’t process the information anyway and all your work will be in vain.