Allomorphs are a group of Bound Morphemes that all do the same thing, like signifying two or more objects.
But what about the plural of "dog"É. It is spelled "s" but is pronounced /z/. And the plural of "hose" adds an "s" in the spelling, but is pronounced /ziz/.
Since we donÅt talk as we spell, the young child must deal with the pronunciation rules. But it gets worse! The plural of "goose" is not "gooses," but "geese". And the plural of "moose" is not "mooses" nor "meese" but simply "moose."
The bound morpheme in that case is nothing, and is designated as /0/, or nothing.
All these bound morphemes which are pronounced differently but do the same thing (e.g, signify plural) are called allomorphs.
The child must not only learn all the allomorphs to express plurals, but also the correct time to use them.
NOTES: This may be more than you want to know about morphemes and allomorphs.
NOTES: A comment (somewhat poetical) about English morphemes and what the child faces in terms of learning it..