"We have beautiful children."
Translation:Nous avons de beaux enfants.
But the noun "enfant/enfants" is still masculine, so it is referring to the word itself rather than the sex of the children. Children would tend to include both boys and girls anyway (which is default masculine when both sexes are represented), because if not, you could just say "nous avons de belles filles"; we have beautiful daughters.
I've found this link helpful in learning which adjectives to place before nouns. http://french.about.com/od/grammar/a/adjectives_4.htm
The general rule for before-the-nous adjectives is Beauty, Age, Good/bad, Size (B A G S). Hope this helps!
Yes, actually "on" does not have all the properties of "nous".
For ex, there is no imperative for "on".
- let's go = "allons-y !" or "on y va !"
In some situations, "on" does not mean "nous", but rather "they":
- on a dit dans les medias que... = it was said in the media that...
Sometimes, it means "you":
- alors, on est en retard ! on n'a pas entendu son réveil ? (so, you are late! you haven't heard your alarm clock?)
- on devrait sortir la poubelle (please put out the rubbish)
Please consider the same sentence in singular:
- Nous avons un bel enfant
The plural of "un" is "des", which changes to "de" before an adjective:
- Nous avons de beaux enfants
Without an adjective or when the adjective is after the noun, this is what you get:
- Nous avons un enfant (gentil) --- Nous avons des enfants (gentils)
"Bel" does not have a plural form nor needs one. "Bel" is used before a masculine singular noun starting with a vowel sound, because the original adjective "beau" ends with a vowel sound which creates a sound conflict: "bo-|-enfant"
In plural, "beaux" ending with an X creates a Z liaison with the next vowel sound and therefore, there is no longer a vowel sound conflict: "bo-z-enfants"