LEGO birthday party!

My sweet little baby Henry is turning SIX YEARS OLD next week. How can this possibly be?? We usually have pretty huge birthday parties for the kids - I'm not talking about pony rides and hired musicians and that sort of nonsense, but we usually invite all of our friends, and really 'do it up' as I've heard they say in the South. I'll post some of the parties we've done later as they were great fun! This year we are having a Lego party for Mr. H., and limiting the guest list to family and the boys in his class. It is always hard having a birthday at the beginning of the year - the big friendships haven't been formed yet, and it is awful to exclude anyone. Luckily, there are 8 boys in his class, and we can invite them all.
We went to the Lego store and bought a package of a million wheels and axles and such, along with tons of loose Legos. Each little guy (and Miss Mae, she keeps reminding me) will get the same amount of Legos and wheels from which to fashion some sort of vehicle with which we will have races! My husband (a self proclaimed 'gear head') spent most of last weekend making a ramp with three lanes, complete with giant numbers shouting out the lanes, for the boys to race their Lego cars. It looks fantastic, and I hope the kids love it as much as Marc does! The boys can then take home their car and leftover Legos, as well as any 'prizes' they win during the races, as their party favors.
As I am NOT a baker, and I usually am focused on the other components of the kids' parties, I've never even attempted to make their cakes. Every year I toy with the idea, then think, "Hell, people do this for a LIVING. Let's buy one." We've had gorgeous ones in the past, including cupcakes with cherry blossoms, a volcano oozing with melted redhots and covered with dinosaurs, and a treasure chest with candy necklaces and gold coins spilling out... But I'm really excited about the idea of actually making a cake this year! I'm not going to go crazy and actually make the cake from SCRATCH or anything, but I'm going to stack pieces into the shape of a couple of giant Legos, and MAKE FONDANT FROSTING! YES!! I've always wanted to, and I think this is a nice baby step - just a big rectangle with a couple of cylinders on top, right?? My friend is coming to the rescue with all of the cake pans and expertise. Plus I've been searching the internet for instructions on fondant. This is a great tutorial: Joe Pastry -scroll down a bit to get to the layer cake instructions.
I looked up a million other Lego cakes, but they are almost always made with a spread cream frosting, and just look messy! I love buttercream frosting, but a Lego should really be completely smooth i think. I'm going to give it a try. if it turns out terribly, I'll run to the store, get a sheet cake, and make a Lego vignette on top!!