Children who study abroad learn to speak foreign languages extremely quickly, especially if they are given the right support. Those who start young enough even acquire accents indistinguishable from native speakers.
But children who have learnt English as a second language (ESL) can sometimes show a lack of linguistic flexibility. This can become particularly apparent in their creative writing and other written work, even once they are writing and speaking accurately.
One effective tool I have found is to have fun with puns and wordplay.
ESL learners often miss out on the jokes native speakers grow up with, but I believe these play an essential part of linguistic development. They rarely feature in educational material but I do not think they shouldn't be dismissed as lightweight distractions. Puns and jokes open young (and older) minds to homophones, homographs and other linguistic quirks. They require lateral linguistic thinking. They often depend on elusive cultural references which are essential for foreign speakers to learn. Above all they bring a bit of fun into the language learning process. Highly recommended!