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