It has been oft-observed that no matter what your first language is, your brain acquires additional languages in the same way (ie via comprehended input, in stages, following a set order, etc).
One study looked at L1 German and English speakers acquiring L2 French. For example, to say do you like to work? in French, you have to say aimes-tu travailler? (literally, like-you to work). In German we say magst du arbeiten? Note that in both French and German, we reverse subject pronoun and first verb to make the question.
It was found that both Germans and English made the same mistakes with subject-verb inversion during question formation, despite German having the same “rule” as French.
This should be comforting to language teachers, who often see “errors” persisting seemingly forever. Why can’t the kids just use plural verbs?, ask Spanish teachers. What is so difficult about the fartitive arricle? whine our French-teaching colleagues. Well, here is a story that may shed some light.
I’m a native German speaker who learned English starting in kindergarten, French in grade seven, and Spanish at age twenty-two. I also acquired a lot of Cantonese from neighbourhood kids around age three, but I forgot it.
In Spanish, when you say I wash my hands, you don’t say *lavo mis manos. You actually say me lavo las manos, which literally means something like “for myself I wash the hands.” The me makes it clear that these are my and not somebody else’s hands.
This “rule” took me forever to acquire. Like, years. And then it hit me.
In German, we have exactly the same “rule” as Spanish. To say I wash my hands, you don’t say *ich wasche meine Hände. You say ich wasche mir die Hände, or “I wash for me the hands.” (The only difference between Spanish and German is where the reflexive pronoun me/mir goes.)
I had to acquire the same “rule” in Spanish that I had already acquired in German, and I had to acquire it the same way that I— and anyone else— acquires it: from the input.
So if your kids are taking forever to say eg estoy bien instead of soy bien, or whatever, relax. Even if their L1 “rule” is like the L2 “rule” they are acquiring— and equally so if there is no similarity— they still have to work through ordered development.
And if there is one lesson here, it might be, resist the urge for grammatical explanations, or cleverly-disguised “practice”, or God help you worksheets, when your kids’ emergent grammars raises your teacherly hackles. Patience, my good sir and madame— there are no shortcuts.