This is usually caused by small worms that develope through playing outside. Simply take a snippet of the childs hair and shred it into a glass of milk, water, juice, etc then have them drink it. Because hair doesn't digest, the hair literally slices the worms and then they are pooped out. The works well with mild constipation of children too.
Clean and dry the child's bottom and apply Beeswax.
keep their anus clean and free of fecal matter aka poop
Anal itching in children can be caused by pin worms. Place a piece of scotch tape over anus over night to look for evidence. The worms will crawl out and get stuck on the tape as they are more active at night time.
If you use fabric softener in the child's laundry, stop for a while to see if the itching subsides.
Traces of laundry detergent can often lead to anal itching, so it is important to get all soap out.
Have the the child eat plenty of foods rich in fiber. Healthy stools are important for avoiding constipation, which can lead to itching.
Be sure the child is wiping properly. Consider having the child use moistened toilet paper to help clean the anal area.
Use cornstarch powder to help keep the area dry.
Change the child's underwear several times per day, and have the child sleep without them.