No (legal) problem taking a passenger on a dual lesson, indeed. They can be very distracting though, especially kids. Puking at inconvenient times, chatting all through the radio traffic. Aviate, navigate, communicate and then take care of your passengers!
If you're flying a half-decent spam can, look for a button called "pilot isolation" or "crew isolation" on the audio select panel. This cuts the passengers from the intercom and is sometimes extremely convenient. A bit rude, but convenient.