I would be really cautious about taking a child that young up the Falls. The rock "steps" are steep for an adult and very slippery, plus the water is flowing down them. I've known several adults, including those physically fit, who have gotten hurt climbing the Falls. One man broke his arm, another cracked his head. So, I'm not talking minor injuries. Think hard about whether it is really worth it. Having to pull your child up the falls will also throw you off balance.
I've done it twice when I was younger. Glad I did, but never again.
Have fun...safely, whatever you decide.