Road riding is good preparation for driving. Street Smarts is the official bicyclist manual for several states, and is available online. If it's not the official manual for your state, it's still helpful, but some details of the law might be different. I've found it *very* helpful, and it should be useful for your older daughter. Roads can be scary places to ride, but understanding the rules helps. Even in places where it feels like every driver is gunning for you :o. Some bike shops and cycling clubs offer road riding courses and maintenance courses. Those might be a bigger value for you than new bike stuff :D.
Your younger daughter is probably not ready to ride on her own most places. Your older daughter is probably good for about a 2-3 mile radius from your house, with good road skills. They both might be more interested if you teach them safe routes so they can go places they enjoy. There's probably a park or playground or friend's house that's a bit far to walk, but a reasonable distance to bike alone. Most kids like being independent. My parents got me into riding a bit as a kid that way, with some simple rules so they knew where I was. I also got rescued fairly often when I got in over my head... meant I learned to remove wheels pretty easily :)
I also find it really helps to let myself do badly. It's ok if I fall. It's ok to not make it up a hard hill. I'll do better next time, and if I beat myself up over the mistake, there won't *be* a next time.

