Hey, since money doesn't seem to be too challenging as-is for some of the people, I have one more suggestion to add for a possible optional challenge:
Good Provider: If you choose to play under this optional rule, you CAN NOT move a Sim out of your household using the "kick out" option. This means that you must weigh how many children you have against how many you'll be able to either keep around in your house, or have enough spare money to move out.
The challenge will be tougher of course early on when your family has so little money it's hard to build a roof over their own heads, much less provide one for someone else, but it might be hard later on too when the town's getting full, and the only real estate to be had for Spares or previous generations is more expensive.
(ones I've suggested previously are here:)
http://forum.thesims3.com/jforum/posts/list/600/39862.page#365915