Of course, it could also be pinworms.
Ah, I remember fondly the day my husband and 3.5 year old daughter and I were at Walmart. I was stading some distance away, her in the basket, my husband pushing, when suddenly she starts screaming "Daddy, my butt Hurts! My butt itches!"
No shortage of attention that day. I kept an entire fruit section between myself and them for quite a while!
We had to treat the whole family for pinworm - used Reese's pinworm medication from Walgreens and followed it up a couple of weeks later with a second dose.