don’t knock lil gay pacman’s! hahaha
your cake looks great, sara. next time, like you already said, use more colouring and it’ll look even more super vibrant. well done!
oh, and here’s how to fill cakes-
Filling Cake Layers
ok, like mehnaz said, i love the idea of using a different shape for the bottom. your cake idea is entirely do-able, btw. you could use a 12" or 10" round and then an 8" top.
since you plan on stacking your cakes, then its important to use lollipop sticks to hold up the top cake and make sure it doesn’t sink into the middle of the bottom cake. take lollipop sticks (you can get them in any baking supply store). cut their length to the height of the bottom cake and insert them into the cake. then place your smaller cake on top of the bottom cake- the lollipop sticks will hold up your cake and prevent it from smashing into the bottom layer. this link will help make sense of this- Using Dowel Rods
they use dowel rods, i use these- Lollipop Sticks - Wilton
they cut easily with a scissor whereas dowel rods require specific cutters.
now for your blocks- you can use a regular cake but you need to put a crumb coat on first to prevent the problems you were having. a crumb coat basically does just that- seal in the crumbs so you can put a second layer of buttercream on it later and then layer on your fondant smoothly.
here is a great video demo’ing the process-
otherwise, the pan looks great- my only concern is how much utility you’ll get out of it after this project is over? if you think you’ll use it over and over, then buy it. otherwise, try the crumb coat technique and see how it works.
remember, PATIENCE is vital when baking. it might take you a couple of tries, but in the end, the result will be well worth it! 
and i think 2" is pretty good size-wise, but decorate one and see how it works for you. you might have to do a 12" base and a 10" top if your son’s name is longer than 4 or 5 initials, i’m thinking. and remember to “glue” your blocks to the cake and each other with toothpicks to hold them together (same idea as the lollipop sticks) but if you put them in through the sides of the cakes horizontally, then they can also won’t shift and move about on the cake and ruin your lovely icing work.
i don’t freeze my cakes before icing them. i do bake them the day before, cool them thoroughly, place them on plates and then cover them in clingwrap and put them in the fridge. i find that does the job for me. i’d give myself plenty of time to make these cakes- bake them all one day, crumb coat the next, and then decorate with fondant the third. the day you crumb coat, you should always have plenty of time to make your fondant and your fondant figurines for the cake.
here is an excellent website for when you want to look up cakes other people have made at home- its full of inspiration and tips.
there are over 19 baby block cakes with pics and descriptions, so this should help you out! Coolest Childrens Birthday Cake Recipe and Photo Gallery
a springform pan is one where the sides come apart from the cake. its primarily for making cheesecakes that can’t be tipped over and out of the pan. so you simply remove the sides and voila! the cheesecake is “out” of the pan. you can get glass bottom spring form pans too so they go from oven to table.
https://www.cooking.com/products/shprodde.asp?SKU=218518
love this idea!
yup!