(By learning I presume "talking" mainly) In my opinion, the best way is to learn by practicing it, if possible with native speakers. That will improve both fluency and pronunciation.
There are various online communities which might come handy. Language exchange option is generally free. Some links-
https://www.speaky.com/https://coeffee.com/https://howdoyou.do (English only)
https://www.italki.com/
There is also an awesome app I used sometime earlier, it's called "Memrise". Memrise is a very good app for increasing vocabulary. There are a ton of courses to learn from and some of them even provide pronunciations.
I taught English in Israel for a year. You have to make it fun, exciting, and relevant.
Pick the 100 most used words in general.
Pick the 20 most used words in categories: cooking, cleaning, eating at a restaurant, asking for directions, food, drinks, etc.
Find movies in the language that aren't boring and are interesting. If it has subtitles, start out with subtitles, but remove them later on, and ask the children what they think the people in the video are talking about. Perhaps provide a list of the most common used vocabulary words that are in each 10 minute segment of the movie, that is, if you already know the movie well.
Have your kiddos listen, listen, listen. Audio recordings of natural conversations and engaging videos are important for context, grammar and tonality! It takes time to develop an ear for the structure and flow of a new language. Immersive exposure is the golden ticket.
For a more comprehensive view, look to Kató Lomb. She was a Hungarian polyglot, translator and simultaneous interpreter who taught herself more than 16 languages. You can see excepts from her book, "Polyglot: How I Learn Languages" and her 10 Commandments of Language Learning here: http://www.lingua.org.uk/lomb.alkire.html
