I have a close friend who has it and lives in Montreal. He has no problems, but he works in the US regularly as a consultant and isn't stuck with the situation where he has months without setting foot in the US. He works in the US close to two weeks a month and the plan has been great for him.
Remember that the plan does not include Canadian data and double check about nights, weekends, and mobile-to-mobile calling. When I had the ATT equivalent of the plan, all minutes I spent used in Canada were prime time minutes.
Also if you aren't under contrary, look at Wind Canada. They have some unlimited calling plans which include the US.
If these are you kid's only phones, also consider the inconvenience of having US numbers over there. Canadians pay more for phone service and I've had many balk about calling a US number when I place restaurant reservations, have distant colleagues call me back, etc. My wife works closely with a Canadian consultant at her firm and they talk to each other by both calling into her company's 1-800 conference number.
My point being you may wish to give your kids some virtual number in Canada. Take a look at VOIP.MS's PAYG plan. $0.99 a month and two cents a minute.