I remember when I was an adolescent boy, there were toy detective kits. It usually came with a little notebook, instructions on estimating height and weight of "suspects," a fingerprinting kit to nab said "suspects," and an official-looking badge so the "suspect" would take the twelve-year-old seriously. Only in my wildest dreams could I ever have imagined this, a telephone bug the size of a quarter and within the reach of most kids' allowances.
Sure, it does need some soldering, but the reward of hearing any and all phone conversations in the house will motivate a precocious child to figure it out. But, of course, landlines are fewer and fewer, so this may only be useful in office buildings and grandparents' homes; the office application is frightening.
Colorado readers should note that unless you are party to the conversation (actually participating in it), it is illegal to record the conversation without a court order. But a little thing like law hasn't stopped us from doing other things, and if you keep it to yourself no one needs to know, so tap away, kiddies!