My Sim has a lifetime want of 'marry off six children'. She has ten children, all of which are married but her want does not appear to have been fulfilled! Why is this? When I married her children, I threw parties for each one and invited her, so she did see the marriage, but I never did anything else. Have I done something wrong?

I have absolutely no idea what you mean in your post. What is a Sims Lifetime Want ?

Tick

When you have a sims 2 expansion pack, sims have a lifetime want, which is related to what sort of aspirations they have (ie family, money, knowledge etc). You can view your sims lifetime want by hovering over the aspiration bar thing (the one that goes red, green, gold etc) with the cursor. If you fulfil this want, the sims stay at platinum aspiration for the rest of his/her life and when they die (they also live longer) their tombstone is golden.

Where they all premade?

I think maybe the idea is the grow each of the offspring from babies to adult and thus marry them.

Its similar to the "have x ammount of grandchildren" aspiration.
Because they could have, say 3 grandkids and need only one more to complete the aspiration, the player could simply adopt a new kid, but that would make getting the aspiration challenge too easy, so instead they need the player to start from scratch.

If you get what I'm saying

Yes I get what you are saying.

No these are sims that I myself created.

I have grown about 10 children of hers to adults and married them and still the aspiration shows no sign of becoming fulfilled.