It does sound like it could be athletes foot, or even eczema. My eczema if I break out on my feet kind of looks like that.
I would not recommend any of the over the counter eczema or athletes foot remedies because they tend to be too strong for toddlers. I would recommend tea tree oil. It's a little expensive but it's a natural remedy for athletes foot and other fungal or skin irritations. Another all natural treatment is Witch Hazel which you can also find at the pharmacy. Witch Hazel is great for all kinds of skin irritations, bug bites, poison ivy, eczema and also makes a great natural skin toner.
After bath make sure the toes are really dry. You can even use a hairdryer on a cool setting to make sure they are super dry.
If that doesn't work I would talk to your pediatrician again, a prescription cream may be needed to help clear it up.