I'm not sure if those wallet size birth certificates are sufficient, they need to have the origional seal on them, so check for that, you still have time to get the ones you may need. As for luggage, just take luggage that you can handle if you have to, I prefer any whith wheeles, they are also easy to store in the cabin, most luggage will fit under the beds with no problem. If for some reason your's doesn't, you can ask the cabin stewart to store them for you.
Here is also a link that has some great information for a first cruise, should help answer a lot of your questions:
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