Condensed Version:
Create 2 families, each with a young adult, teen, child, and toddler with randomized traits and unique lifetime wants.

Move a family in; then, the merge the second family with the first.

Play on normal aging.

Traits assigned when a child ages can only be given once. (Randomly assigned traits do not count.)

1 point each (or half point if playing with "cheaty" mods):
toddlers: skill maxed/skill book read/friend made
children: friend made (-toddlers)/different skill learned/earn and maintain "A"
teens: friend made(-toddlers & children)/skill learned/honor roll maintained 3 days/day worked
10 points each different lifetime want achieved.
-1 every Sim day that passes.

The long, drawn-out explanation:
I really hate playing legacies because, for me, the challenge is taking a poor family and expanding their home over time. By the time the first generation has reached their lifetime wants, the house has everything I could possibly want and definitely everything they need. I do like playing challenging families though and accomplishing goals, so here is a challenge that doesn't require having any more children and playing through multiple generations.

Create a male and female young adult. Randomize the traits and keep whatever you roll the first time. Based on the traits, pick one of the five lifetime wants available. Then, spawn three children: one teen, one child, and one toddler. With all three children, randomize the traits again and keep what you roll the first time. Now, delete either the father or the mother and save your family, adding the relationship lines of the children to the single remaining parent. This family will start life with 18.5K.

Do this again, creating a second family in the same fashion with random traits. Ensure that each lifetime want chosen hasn't been selected already. Move in one family and pause the game. Then use "edit town" to "move in" the second family with the first by "placing a copy" of the second family (in the bin) on the lot with the first, merging the two families into one. Return to your game and save it with whatever name you wish.

You now have 37K to build a decent-sized house. You should be able to afford expensive beds for everyone, an unbreakable toilet, a nice shower or tub, as well as a good refrigerator and stove. You can also get a counter, sink, table, chairs, a potty chair, and other inexpensive items.

Your first goal is to train the toddlers to walk, talk, and use the potty before they grow up to be children. A secondary goal is to let them gain maximum toddler skill in logic and music. A third goal for the toddlers is to read books left on the floor to get a head start on other skills. Points: 1 point for each skill preparation taken as far as a toddler can go before aging.

Toddlers should be friends with everyone in their immediate family, if you remembered to add the relationship lines in CAS. Now, try to make them friends with the four members of the other family. Points: 1 point for each friend made in the other family before aging.

Children will want to make friends with everyone in the family as well as a school mate. For each friend they make before growing up (with the exception of the toddlers because you can't count points twice), give them 1 point. Also, if they managed to get and maintain an "A" in school, give them another point.

Children can gain skill points in several different areas. Since it is good to be well-rounded, try to get one skill point in each area possible. Get 1 point for each different skill point gained as a child (and for getting as far as possible in cooking and mechanical).

Teens can be a great help taking care of toddlers and being home for children after school. However, they will want a part-time job, which will add to your family funds, but seriously deplete their fun and keep them from socializing and skill-building. For every day they work in the same part-time job career, give them 1 point. Also give them 1 point for each skill point gained and each friend they make (with the exception of the toddlers and children in the family). Also, if they made and stayed on the honor roll for three days as a teen, give them another point.

Once Sims are young adults, there are no more points for friends or skills. You need only concentrate on accomplishing your lifetime wants. Accomplishing each different lifetime want is worth 10 points.

When children age up, either naturally or with the aid of a birthday cake, you can choose the trait rather than selecting a random one. However, you may not pick a trait that you have already picked. (This does not count the randomized selections.) For example, if you gave one teenager the "genius" trait when aging to young adult, none of the other children may be given that trait. It is okay, however, to give a trait that was already randomly assigned (in CAS) to another Sim.

The challenge ends when the last of the children has achieved his or her lifetime want (or dies without achieving it).

Set aging to normal for this challenge and deduct one point for every Sim day that passes. You may move Sims out as soon as they accomplish their lifetime wants. You may also move Sims in, but achieving their lifetime wants does not add to points.

If you play with mods that let you skill-build faster, then get only half a point for each skill affected.

I know this is an old thread, but I've started this challenge and looking forward to seeing how it goes.

My family:

Carol Martin and her daughters Marcia (Teen), Jan (Child) and Cindy (Toddler)
Mike Simly and his sons Greg (Teen), Peter (Child) and Bobby (Toddler)

Yes, I'm soo original with names... NOT

That's okay, and now Mike can actually be a designer as he was in the show if you have Ambitions. You could even get a simbot named Alice. Have fun and thanks for commenting. Let me know how it goes.

You do know that in an unmodded game it is impossible for a Child to achieve the "honor roll for 3 days" wish? The Child Lifespan lasts 7 days, only 5 days of those are schooldays, in order to get on the honor roll a Sim must have an A for 3 days in a row. Simple Maths

EDIT: Obviously this only applies if your playing "Normal" Lifespan or lower

Okay, I changed that to just getting and maintaining an A. It is possible for children to make the honor roll (just not stay on it for 3 days).