I've found that you can get a new recipe from a natural cook or chef every day. You can only get one a day, but you can definitely get one every day.
This is also
per natural cook. If you know two natural cooks, you can get two new recipes a day. However, Trade Kitchen Secrets does not work with natural cooks living in your household.
I usually have my sims move into the lot next to the Frio brothers in Sunset Valley. Jared is a natural cook, your sim only needs to know that he has that trait and then they can Trade Kitchen Secrets once each day. This is how I have my sims learn all the recipes. When your sim cannot tell that he is talking with a natural cook, if he talks about cooking enough, the option to trade kitchen secrets might appear... but rarely.
Also, I haven't seen noted that a perfect pie that a child cooks in the oven gives a
+50 moodlet. Neither a perfect muffin nor a perfect cake give that big of a boost to a sim's mood.