hi, welcome to avs diy, you are in the right section.
at that age, they are going to want something that makes as much noise per dollar as possible.
for the "mains" i would take a look at a full range driver, something like you might find in a bose cube, that has a "reasonable" frequency response and just run them with no crossover or anything off of whatever stereo set they have. you could probably just find a used stereo system with cheap speakers for the same money and less work to be honest though.
to make the system special, build a single fold tapped horn. this will "blast" bass and truly make the system something that you can't buy in a store for anywhere near the money. for a teenager, big bass adds more fun factor to a system then any other upgrade.
a single fold tapped horn is a very easy build and doesn't require either an expensive driver or amp. when they fire it up, it will make them laugh out loud and next thing all their friends will be building tapped horns. :-)
this is an example to give you an idea of what this is.http://www.cowanaudio.com/images/th35dwg.gif
several folks here are familiar with the drivers that give a great bang for the buck in a tapped horn application. the only other thing that you will need is a low powered amplifier with the appropriate connections to whatever source you intend to use.
there are some more pictures here (scroll down):http://www.cowanaudio.com/th.html
the larger the enclosure, the lower and louder the unit will play. as you ask questions, shoot for something with about a 35-40hz "corner".
this is a great project to teach your nephews all kinds of lessons, particularly about safety as well as the extra satisfaction that comes with building something.
good luck with the project and i'll try to help as much as possible.