First thing to do is make a copy of the babyrez.dll file.  Keep the copy safe incase you mess up the original one.  Next, open it in the hex editor.  Now click "edit", then click "find."  In 4.0, you'll then have to click "text string."  But you won't in older versions.  Now type in the babyz rez lnz for the type of baby you want to hex.  Note that the lnz is LNZ in lowercase.
 
White Girl =  Wg.lnz
Black Girl = Bg.lnz
Tan Girl = Brg.lnz
Asian Girl = Ag.lnz
 
After typing in this search string, you will be brought to the section that edits the type of baby you want to hex.  If you want to hex more than one colored baby, you will have to edit the lnz section for each of those babyz.  Scroll down to the ponytails section to begin editing the hair.
This is where you can edit the two ponied babyz hair.
This is what you can edit.  Number 46 is the color of the hair tie.  If you don't like the normal hairtie, then change the color to anything you want using the color guide.  30 is the size of the hair tie.  If you want a bigger hairtie, you can raise the number to say 40, or 50.  0 is the fuzziness, or fullness, of the babyz hair.  Raising this number adds more fuzziness.  The rest of the numbers highlighted in pink are the sizes of the bobbles that make up each section of the babyz ponytail.  Raising these numbers make each bobble larger.  I recommend keeping the number raised by consistant to make even hair bobbles.  For instance, add 10 to each number, or add 20 to each.  Since there are two ponytails, everything occurs twice, as seen above.
For a baby with hair like Jenna's, add 20 to each bobble, and make the fuzziness of each ball 50.