I find it absolutely adorable when Hila slips Hebrew into her everyday English speech -- ("Slicha chair," she says as she moves her chair out of the way) -- but because I want to make sure that she speaks both languages fluently and separately, I usually correct her Hebrew by repeating back what she said in English.
There are a few exceptions, though. A few words that I think make more sense in Hebrew than in English. There are a few, but the only one that comes to mind right now is the word "tapuz."
We're teaching Hila her colors (at home in English and at gan she's learning them in Hebrew), and I think it's very confusing that an orange (the fruit) is called by its color. Why doesn't it get its own name like every other fruit? And anyways, tapuz is such a nice sounding word and Hila says it beautifully.