Her children will be half-alien, half-human. They won't be able to disguise themselves, but, if you're lucky, they will look human. They may also have some alien abilities. The only alien-Sim hybrid born into my game so far does appear completely human, but he doesn't have any alien abilities (though he can do the secret handshake with other aliens). Of course, there's always the risk that any hybrid children will appear more alien than human, such as in the image @Samtaplet posted. If you don't want to take that risk, I would recommend staying with the guy but adopting children instead.