Now that he's four years old, my little Léo has figured out the trick for speaking both French and English: The overlap in vocabulary is huge, so if you don't know the French word, just say the English word with a French accent (and vice-versa), and you'd be surprised at how often that works. (For his early language development, see bilingual babies).
Léo's English doesn't seem to be affecting his French very much, but he's definitely enriching his English by translating stuff literally from French, with some funny results.
The first one is "Est-ce que...?" That may look daunting to non-francophones (it means "Is it that...?"), but it's two simple syllables (pronounced "Eska") and it's just a trick for turning any statement into a yes/no question. We don't have any such convenience in English (we have to do that whole messy inverting the subject and verb thingy), so Léo just carries it over wholesale, producing questions like "Est-ce que it pokes?" That question was meant to be something like "Does it sting?" but was clearly influenced by the French version of the question: Est-ce que ça pique ?
Another funny one is the redundant pronouns. Instead of just using stress for emphasis (as in "I saw him" vs. "I saw him"), the French like to throw in redundant pronouns (as in "Moi, je l'ai vu"). Léo likes to do this in English too, so rather than saying "I did it" he'll say "Me, I did it."
But I think my favorite Léo-ism is "But sure!"
In English, it's tricky to contradict a negative. If someone says "He's not here," you could say "Yes, he is," or "Yes, you're right" -- so "yes" alone can have two opposite meanings in response to a negative. French gives you three choices: oui and non (corresponding to the usual "yes" and "no"), plus a third option si -- just for contradicting negative statements! Cool huh?
Once you've gotten used to using si, it's hard to go back to not having it (you'd be surprised how useful it is!), so Léo has decided to equip the English language with its own version of this useful word: "sure!"
And since why say oui, non, or si when you can say mais oui !, mais non !, or mais si !, Léo translates the mais as well.
So the correct response to a statement like "You can't have any" in Léo's language is "But sure!" :D