In particular, I would like to change the gender of some of my Sim children. Yes, I know you can save and restart the birth sequence until you get the gender baby you want, but there must be some way to change them after their born, surely.

Is there a cheat or a hack or some other technique for doing this?

Im not sure, but I think you can do this by using Simpe.

You can, but to my knowledge they will still look like the original gender. They won't change from male into female. Only their flags change.

I figured out HOW to do it, although it's not easy. There was a confusing thread on the SimPE site that had some suggestions that were true and some that were just wrong. The best explanation was HERE

I'll try to give a slightly more thorough explanation. This is such a vital tool that it probably ought to be in a FAQ or Sticky. Even better would be if somebody could make a tool or hack to automate this.

1. Open up the character in SimPE that you want to change.
2. In the Overview panel, change the line that says "Treat as male () female()" to whatever you want it to be. Then click "Commit".

You are NOT done yet. This is where most of the tutorials screw you up. This changes hardly anything. The next part is trickier, and if you blow it, you could destroy your game, so be careful.

3. Click on the "More" tab in the Plug-in. You should get a number of technical options, including one saying, "Open Character File." Click on that.

The Character plug-in view should go away, and you should see a bunch of scary looking technical resources at the top-right of your screen. You need to change a few of these manually.

4. Click on the resource named Age Data. When you do, a new plug-in screen will appear at the bottom right called The CPF Editor, which allows you to change some of the values inside the Age Data resource. The one you are interested in is a line that says:

Gender (dtUInteger) =0x00000001 (If the character is female).
or
Gender (dtUInteger) = 0x00000002 (If the character is male.).

Click on that line, and change the 1 to a 2, or the 2 to a 1. Then click Commit.

5. You need to do this four or five more times. Look at the resources again. There should be some entries that say Property Set. Click on them, one at a time and change them, just as you did with the Age Data resource, changing the Gender from 1 to 2 or 2 to 1. After each change, click Commit.

I counted four or five of these when I did it. It may be a variable number -- I don't know. Make sure you get all of those Property Sets.

6. Click on File and then Save And exit if you want.

It would be a good idea to save a backup before you do this the first time. I got it right the first time, but I'm a used to diddling things.

Any modders out there... please consider making a tool or mod to automate this!