The helmet is incidental, IMHO. You're not in control in a fall, even top-roping, so there's a good chance you'd crush the child. Then there's the impact as the rope comes taut, and the load rating of the baby carrier or whatever she's carrying the baby in. Okay, a tight top-rope means you won't fall far, and the impact will be light, but I don't think this is a great idea.