A couple of disclaimers first:
Not religious myself. Am agnostic verging on atheist.
If the couple marry in a secular ceremony (NOT involving religion), well I don't think they are being unreasonable not to invite children. After all, all they say is the legal bits with perhaps a few personal words in their vows. Fair enough.
But given the fact that most religions are based upon concepts of community, family life, why do people think it OK to bar children from such events when they are a vital part of what a religious wedding is supposed to be about?
Also, is it practically possible to ban children during wedding service (it obviously IS for private party/functions after wedding)? Cannot anybody walk in off the street to witness a church wedding? I've been to a lot of weddings where casual passers-by are at back of church just looking. Or can the Vicar lock the door?
Isn't the fact that everybody (not just guests) can witness vows a vital part of it for most religions, anyway?