foster-child vs tiddler

foster-child and tiddler both are nouns.

Word	Noun	Adjective	Verb	Adverb
foster-child	Yes	No	No	No
tiddler	Yes	No	No	No

As nouns, tiddler is a hypernym of foster-child; that is, tiddler is a word with a broader meaning than foster-child:

Other hypernyms of foster-child include child, fry, kid, minor, nestling, nipper, shaver, small fry, tike, tyke, youngster.

foster-child (noun)	tiddler (noun)
a child who is raised by foster parents	a young person of either sex