The idea sounds good … 12" and an 8" on top? It will leave a good amount of ‘ledge’ (on the 12" cake).
How many animals were you planning on putting on the 8" cake? You won’t be able to do many, I’m guessing around 4 max. On my 10" cake, I was able to put 6 animals, plus a candle in the centre … so make sure you figure out the size you want to use first.
I am a bit confused as to where you plan on putting the building blocks? I think it will be a bit difficult to make 3 dimensional blocks. But, SGC might have an easy way. What if you made flat blocks out of fondant instead and have them going across the side of the 12" cake? I think that might be easier …
Freezing and icing the cakes is super easy. Just freeze them the normal way and defrost them the night before you plan on prepping and decorating it. Leave them in the fridge the night before. There won’t be any water leaking, I’ve done this many times and have never had any issues.