If you just want to move sims out and have some sims stay behind, all you have to do is have an adult sim click on the newspaper or computer and select "find own home". You can select the sims you want to move out and they'll be moved to the family bin. However, no matter how much money they've earned, they'll start out with 20,000 simoleons. The family still living in the original house will have all the cash.
If you want the move the whole family, you have to go the neighborhood view and click on the Families button on the user control panel, then click on the Move Families icon. Click on the family you want to move, and they'll be returned to the family bin. You can move them into another house from there.
If you have Nightlife, you can use another cheat to give them money before you move them into a house. It's a bit of a quicker method over buying a cheap house, using motherlode, moving them back to the family bin and buying a better house. You just open the cheat box (in the neighborhood screen) and type in (without quotations) "familyfunds NAME xxxx" Name=the family name and xxxx=the money you want them to have. For example. If I have the Baxter family in my bin and I want them to start out with 500,000 simoleons right off the bat, I'll type "familyfunds Baxter 500000" into the cheat box and voila!
There's a list of cheat codes
here, if you're interested.
