The suffix -en indicates a number of things in English. It appears at the end of past participles of many strong verbs (eaten, written etc); it ends many verbs formed from adjectives with the sense 'to make ... or to become ...' (deepen, brighten etc); it can indicate a diminutive, eg chicken, kitten (maiden is also in this group); and it can form an adjective from a noun, particularly a noun denoting a material (woollen, wooden etc).
In Old English -en also denoted the feminine form of some masculine nouns. So, gyden was the Old English goddess, mynecen was nun (from munuc, monk) and wylfen was a she-wolf (from wulf). The only surviving word in modern English of this type is vixen; the Old English was fyxen.
In Middle English -en replaced the -an of some Old English plurals. The only surviving member of this group is oxen, the plural of ox; in Old English oxa was one ox, and oxan the plural. A thousand years ago there were others: name, naman; tunge, tungan, eg. Children and brethren do not belong in this group, although these are plurals ending in -en. Childer was once a variant form of child, and brether was a variant form of brother, and an -en was later added to these words to form the plural.