Many of the colonies and states had a position called Overseer of the Poor who collected funds, oversaw foundling children and poor adults. The laws differed state to state, but in some instances children were put to indenture until 21 years of age, in the hopes they would learn a trade (this is true of Virginia). I don't know of any comparative information about a foster versus indenture situation.

Here's a good example of an Overseers duties: http://archives.delaware.gov/collections/aghist/2230.shtml