This is basically the challenge made by GlitterForSimmers but I tweaked some of the rules to my liking.
You can make one sim or two, like in GlitterForSimmers version. It's easier if you have husband and wife. Both must be adults, to make the challenge a bit harder. They must have one child who has to be a teen.
Make a small house on a minimal of a 20x20 lot. You're poor and can't afford a lot, but you need these kids! ... Don't ask why.
Both parents and your teen need to have a job. You need all the money you can get.
When you start the game, you must set money to 100. You need a starting. Or for maybe hard mode, have 0. I don't care.
If your parent sims die, you must restart the challenge with your eldest child. Move out the other children if you want, it doesn't matter.
Only one child should be made in CAS. NO cheats, unless it's to either lower money at the start of the game or move objects.
You must complete every pregnancy with no mods. 1st trimester, 2nd, 3rd, labour. NO MODSSSSS!
To have 19 kids. Your maximum is 20, because you already had one child at the start of the game.