At that age it is pretty much irrelevant what your baby may want to become. It is not a concept they will know or have. They have no idea what a witch is, or a mummy, or a zombie.
The important thing is for you, your family and friends to have fun, and perhaps have something you can look back on in future and see that the babies looked cute/funny/whatever in photographs.
To be honest, if you buy a baby Halloween costume, it will be cute - shops have no mileage in trying to make babies look horrible. As they get older that may change (my 12 year old looked an absolutely hideous zombie at her school Halloween party the other night, but that was her choice)
What you will want to do is make sure they cope with all the scary folks they will meet at a party or trick or treating :-)