If a child (or adult) knows the other speaks both languages they will switch very naturally between them, even within sentences and words. A fave in our family is Eng/Ger: I have geunderstanden.
Yes, in a way, there are specific situations. Sometimes a child will use a word in the other language if they don't know it in the other or if can't retrieve it fast enough for the conversation, even if they know the other person doesn't speak that language. Adults may do this too, for example, if their professional vocabulary is from another language, i.e. they speak different languages at home and at work.
I don't get money for the questions, only for people clicking on the ads.