Is it safe or unsafe to climb an unsecured ladder that has slip-resistant feet?

Of course not safe.
When a ladder is slippery it means it has no friction and you would obviously fall down from it. Its something like walking a surface with oil spilled all over it. You will not be able to maintain the balance and it may cause serious injuries. There are many more things which you should avoid in order to stay safe including choosing the right ladder for the job and tying the top and bottom of the ladder to fixed points.

Stand or kneel behind the child and wrap your arms around his or her waist. locate the navel with one or two fingers in the middle of the child’s abdomen. just above the navel and well below the lower tip of the breastbone