Thank you Bri!! Is there a make needs static cheat?
Actually, there is, and is similar to the one in Sims 3.
Now, it has been a while, so I might be a little off, but you need to enable cheats (as mentioned above). Then press Shift + Click (with the mouse) on the Mailbox. A series of things should pop up from there. From there, look for the one that says Fill Needs. Choosing that, there should then be an option to make Needs Static.
There is an alternate way, I believe. Shift + Click on the mailbox, and choose, I think it is called Spawn, and there should be something there called Sim Modder. If you choose it, then a small baby should appear. Clicking on this should allow you to max motives anytime.
However, the easiest and one I recommend are two codes.
First, use this:
maxmotives - This makes everything maximum
motivedecayoff - Turns the needs falling off. You may have to reuse this if a new family member is added, though
Something to bear in mind is if you are trying to potty train a child, you can't have the appropriate Need fulled (the baby has to sort of er...want to go...otherwise she will just drop off the potty)