This is a neat idea, and I also agree that there should be various options. You could use the computer to "start a trust-fund for ____" with various starting amounts. You could then choose to set a weekly deposit of various amounts, which could be discontinued or altered later, and then could also manually add any amount you liked at any time. Then, when the sim in question moved out, the whatever amount was in the fund would automatically go with them and be added to their new household funds. Or, even better, if we could manually "move out" in game,you could have the option of unlocking the fund when you did that.
I'm really in favor of anything that allows for the transfer of funds when a kid moves out.