No change in plural form
There is no change for most neuter or masculine nouns that contain any of these in the singular: -chen, -lein, -el, or -er. There may be umlaut changes. For example: "das Mädchen" (the girl) becomes "die Mädchen," and "die Mutter" (the mother) becomes "die Mütter."
But isn't "die Mutter" a feminine noun??
The nouns with "-chen" and "-lein" are always neuter and forming their singular form always requirs a change of the Umlaut. And the plural forms are identical.
die Mutter => das Mütterchen, das Mütterlein das Haus => das Häuschen, das Häuslein der Baum => das Bäumchen, das Bäumlein