I've been diagnosed with being low on vitamin D. Since I started taking a 1000 IU supplement that my doctor recommended, I've seen much reduced skin issues.
My son has skin issues. We have to slather him with creams like Hydrolatum to keep his skin in good shape. We have a prescribed steroid cream for when the Hydrolatum isn't enough. He's had allergy tests and they have not pointed to his skin issues being an allergy issue. We usually give him a 400 IU vitamin D/day supplement.
How much total vitamin D does a toddler usually need, including what he gets from sun, food and supplements. I know the 400 IU recommendation is on the low side. We live in the US northeast so for large parts of the year, he gets very little vitamin D from sun. Of course, he gets some from milk and the supplement we give him, but I suspect he might need something more like the 1000 IU my doctor recommended to me. Doctors we've seen haven't been specific about the total amount of vitamin D my son needs.
I know about the ODS Vitamin D page.