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??
Feminine nouns normally add an -n. For example: die Mauer/die Mauern, die Schwester/die Schwestern, die Schulter/die Schultern.
Some exceptions: die Tochter/die Töchter, die Mutter/die Mütter.
The plural articles for neuters are the same as for feminine nouns and neters. So “die Mütter, die Mädchen” are correct. Indeed, das Mädchen is grammatically a neuter. Die Mutter is a feminine noun.
I know. Confusing thing is the instruction said "neuter and masculine" but the examples are neuter and feminine nouns.
-er ending is much more common for masculine nouns. I guess we can consider die Mutter, die Mütter as an exception: feminine and still it only gets an umlaut.
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