I would like to make small permanent sandcastles for wedding favors...

don't know that it would work but what about adding some glue to the sand or cement. Check out a craft store I'm sure they could help or even a hobby shop or ceramic shop. Hope you work it out.

Why don't you get some beach toys (like small sand castle plastic designs) for the sand. Mix the sand (you can buy it at WalMart) and glue and fill the molds and let them dry. Would help to have a couple of sunny days. Then unmold them. You could even use your imagination and use food coloring to make different colors. But, I think I would prefer the original sand color.