I'm a firefighter, my wife's a paramedic...we're away from our kids (not simultaneously) for 24 hours each shift.
Facetime is a wonderful thing for when one of them needs, well, a little facetime with whatever parent is at work. They get to chat with grandparents as well.
We bought a couple of refurbed iPod touches, put them in otterboxes, threw a few apps on them, and handed them over. They can facetime us as long as they have wifi (at our house, family, close friends), their texting is limited to iMessage and locked down to the existing contacts...this way they have an opportunity to learn proper etiquette and manners about the phone and texting and pictures.
They're 7 and 8, have had this for two years, and they're not little tech junkies. Also, I'm not paying an extra $40 per month per kid for connectivity that's only occasionally necessary.