What is the cause of cracked corners of your toddler's mouth lasting about 4 weeks per year and is it a yeast infection or too much acid from juice?

It sounds like he probaly just needs some lip balm. Put it on several times in the day and put it on at night when he/she goes to sleep and can't lick it off. My daughtet has a bad habit of sucking her bottom lip and it leaves dry skin all around her mouth the lip balm helps keep it from getting to bad. Sounds like an oral yeast infection to me. A dab of Miconazole Nitrate (aka "Monistat") would likely cure this condition in 1 - 2 days. I've gone through months of this in the winter, tube after tube of every kind of lip balm, and it was the Monistat that finally cured me.