Wow, that's a very impending birth! Best of luck!
The only problem I've had with paperboard/cereal boxes is when I've applied too much liquid (paint or runny glue), they curl up badly. It's easy to avoid that, though.
One rule of thumb I've heard is to hold the finished card out straight by one corner -- if it flops over, it's too thin. If it stays upright on its own, it's fine.