When Boy1 was little and just starting to speak both languages, he mixed things up sometimes. He never said 'that' in sentences like 'Papa told me that I could have some more cake.' He would say "Papa told me que I could have some more cake." The que is easier to pronounce, I figured he'd figure it out eventually, which he did, when he was about 3 1/2. He also never said 'for' and used the French pour instead, although never at the beginning of a sentence. So he would say, 'for me!' if I asked him who the first crêpe was for. But he always said things like, "This book is pour you." I'm sure it was all very logical and systematic in his head. And again, after a while, he figured it out. Interestingly enough, when learning to count in English, he always skipped four. 1,2,3,5,6, all the way up to 10, but never including a four, and never substituting anything for it. Four didn't exist. Poor four.
Boy2 is three. His English is quite good and much more advanced than his French. He says things like, "I already peed!" "My pillow belongs here!" and "Of course you can have a hug." And my favorite, "You can't touch my cookie Mama. Because I said so, and I mean it!"
But, just like Boy1, he always leaves the 'four' out when counting. And he always says pour when it's not found in a sentence initial position. What is up with that? Is there something inherently objectionable in the words for/four? I am a great believer in the wisdom of children's intuitions so I guess there must be.