I'm assuming you want a large, flat-screen affair for the main monitor, and possible some smaller monitors clustered around that? I guess it really depends on how much work you want to put into the project. If you want to do it from scratch, probably the simplest way to go about it would be to find, or make, a desk of some sort and some thin, or box-like, rigid rectangular shapes (wood, plastic, etc.) for the monitor(s) and then assemble them together. After painting it up, you could print out some appropriate Batman-stuff (maps of Gotham City, dossiers for his Rogues Gallery, etc.) and glue those onto your screens for the monitor images. Personally, I'd probably just fabricate everything from scratch, probably from cardstock, although, as you indicated it's for your son, you'd probably want something a bit more durable than that, as boys tend to be rough on their toys.
If you're not handy at making your own stuff, I imagine that you could probably get some computer equipment pieces from other toy lines, like G.I.JOE or Star Trek, that could be assembled into a Bat Computer terminal. It might be worth your time to take a trip to your local thrift stores and scrounge around in the toy bins, you may very well find exactly what you're looking for [I found a Barbie computer monitor, with screens that can be changed via the flick of a lever (similar to MOTU Man-E-Faces) at one myself last week, which, even though it's larger than Playmobil scale, would certainly work].
It'd also probably be helpful to look at some of the examples of Bat Caves, here at Figure Realm, to get some ideas for parts and components as well.http://www.figurerealm.com/viewcustomfigure.php?FID=12996http://www.figurerealm.com/viewcustomfigure.php?FID=12996http://www.figurerealm.com/viewcustomfigure.php?FID=22779