One of the skin's jobs is to keep the outside world out. A baby's skin acts more like a sieve, with a structure that allows more substances through than an adult's. One reason is its looseness, owing to fewer elastic fibers, which in adults, form a thicket and make the skin dense.
The top layer of a baby's skin, the epidermis, also doesn't attach as well to the layer below, the dermis. The two layers have fewer microscopic seals, called hemidesmosomes, between them. So unlike an adult, whose epidermis and dermis are tightly sealed together, a baby's epidermis is easy to get past. And finally, a baby has more fat under the skin than does an adult, so some fatty substances, like steroids, more easily sink in.
To find out how you should specially treat a baby's sievelike skin, read on.