Take them to the range and let them hit a few balls. Then you can explain a few things to look out for. At first, they should just enjoy watching a ball fly.

Try to find a golf camp or clinic. It's a great way to introduce them to the game. They usually keep the focus on fun, while teaching them the basics. If their interest grows, then try to hook them up with a good instructor.
My personal experience is softball and golf don't mix well. My daughter was a tournament softball player for many years, and tried golf on and off but the different swing mechanics conflicted so she stopped golfing. At 20 she was done with softball, and started golfing again. Didn't take long to adjust her swing, and now she can outdrive me all day long. I still get her on the putting surface though. It was a long wait, but nothing beats being on the course with your kid playing golf. Greatest father-daughter sport in the world.

If they enjoy going to a range, if they want to get more, many courses offer summer camps for juniors. Basic courses, not like moving to FL for career development.

I'd see if there are any clinics or group lessons available. It's better for them to learn from someone who can teach them the basic fundamentals correctly before they get bad habits ingrained. These clinics should be keeping it fun for the girls and group them by age, sex or both. Plus, they will have access to clubs rather than hit your set which might be too long, too heavy, etc. for them.

and who knows? maybe they will come back and teach you a few things!

I would take them to the range to start and let them just swing away and try to give a few little pointers here and there. Give them fun little challenges "if you hit it close to that marker you win" etc. then start taking them to the course and letting them play when they wanted.

I second the above, a local clinic or camp is a great start. Definitely a different swing than softball so learning good fundamentals early is important. Another good thing if you have the room is chipping around the yard. Pretty easy to make a short course chipping to trees or putting a few stakes in the ground. Extra points if you have some slope so they can learn uphill and downhill lies.

Happy to hear! My son just started showing interest this summer. Great game to enjoy with the family.