Let them be; they’re creating a new language, like the Ladino of America.
The question is whether they are receiving an adequate education and practice of the English language. If so, then at least they have a language that speaks to them. It may be tinged with Yiddish and Yeshivishe, but language is a framework for life.
There are some things that cannot be translated into English that only they in their time can communicate by merging the languages.
For the sake of Yiddishkeit, perhaps the language barrier between their casual conversation and that of others actually keeps their ties with Judaism strong. They cannot rid their Yiddishkeit from their tongue.
What’s the difference between criticizing them for not speaking 100% English and previous generations doing all they can to assimilate? Aren’t you making the same argument that the Reform movement made to enforce English?