I almost never correct my kids when they make grammar mistakes or other language errors. That's probably wrong of me, but I just think it's so cute when they come up with their own words and grammar -- like the word cracre-de-paix, which they invented long ago, but still use sometimes. They also say "virgule" instead of "vehicle" (virgules are a very important subject of conversation at our house). Sure, I could correct them, but they'll figure it out eventually.
One strange mistake that Leo makes is that he essentially doesn't use pronouns to distinguish people by gender. In Leo's world, everyone is he/his/him -- even people he clearly knows are female. Originally he was referring to everyone as she/her, but I imagine that he made the switch after male companions complained. I'd chalk it up to his bilingualism, but he makes the exact same error in both languages. (There are other cases where he'll translate literally from one language to the other, and it doesn't quite work...)
I just saw a new article on the benefits of bilingualism (hat tip too many tribbles). Whether it's beneficial or not, it's certainly cute. Remember the three dolls from the other day? Well, Leo decided that Mimi's name in French is "Moi-Moi." There's a certain logic to that. Then they decided that Jack-Jack and Incredible should have French names as well: "Jacques-Jacques" and "Incroyable." :D