i used to nanny for a baby that was allergic to everything, and had the most ungodly sensitive skin ever.
she had this fancy german baby soap that i don't know what it was called.. but when that stuff wasn't around we used this:
it's arbonne baby wash, and it's really light (wont leave a film or residue or anything at all, ever) and unscented.
or maybe just baby soaps in general, like the sort you can get at the drug store