They certainly could hurt themselves, or they could fall and NOT get hurt, or they might not fall at all. Mamas are wired to worry – it's our job. We are capable of imagining every possible terrible consequence on behalf of our happy-go-lucky kids.
And amazingly, most of them grow up relatively unscathed in spite of many falls and near-misses. So another part of our job as mamas is to take a deep breath and let go a little more each day as they become more imaginative and capable.
Your girls sound pretty precocious, and that is something to delight in as much as be terrified by. Think about how different you might feel if they were boys – might you have an expectation that they would take more risks and get more owies? So, why not girls, if they are up to it?
When my daughter was their age, she would want to climb or have other daredevil adventures. I didn't try to stop her unless she was in genuine danger. I would tell her, "Okay, you can do that, and I want you to know that xyz could happen. How will you feel if that happens? How can you keep it from happening?"
She would have a chance to think over the outcome, and would usually let me show her the safest way to do something, or even try a less risky version until she got good at it. She was always physically strong and well-coordinated. I doubt that her body tone would have been as good if I had kept her from extending herself into new territory.
By the way, my 27 mo grandboy kicked a chair out from under himself while leaning on the table last week. It looked like a terrible fall, and every adult in the room rushed to administer first aid. It scared him, and bruised one hand a little, so he cried for a few minutes. And then he was fine.