My five year old daughter has a habit of playing with my elbow, that is, the skin over the elbow when its extended. She developed this habit around the time I weaned her off breast-milk, at about 13 months of age. Since then she has had the need to do this at the same time as sucking her own tongue. It’s kind of weird, but I guess it calms her, as she falls asleep doing this, and even when she is idle, she resorts to it unconsciously. Its very irritating for me, or my husband, as its annoying, not to mention that it turns the elbows sore! All our efforts to get her to stop have failed.
Your daughter’s desire to play with the skin on your elbows initially probably started out as a comfort to her as a baby and then over time grew into a habit, one of which is hard to break after 4 long years. Many kids develop habits, such as thumb sucking, hair twirling, picking fuzz off blankets, etc. as comforts as young children. Since your daughter is 5, she is old enough now to start to cognitively understand that it is not a socially appropriate behavior and you can even explain to her that it is making your arm sore and hurts you when she does it. Don’t scold her for it, but calmly make her aware of when she is doing it and I suggest first channeling it to something similar in texture that is more appropriate to touch…since this sense of touching seems to soothe & calm her for sleep. Some kids develop oral fixations, but it sounds like your daughter has developed a tactile fixation.
Have you tried using something such as the water filled snake toys that sort of feel slippery and have the texture of skin when touched? Perhaps try easing her into using a different more appropriate outlet that gives her the same feeling of comfort, but does not use your elbow as the target. Plus, something like this, she could carry with her. When she starts to touch your elbow, just remind her to get her toy and that she can touch that, but not your elbow. Praise her anytime she is not touching your elbow, drawing attention to her positive behaviors and not to the bad habit.
This is a great article on breaking bad habits in children that may give you other ideas on how to stop the behavior.