Something that will help a lot, if you haven't already done this, is to be persistent every time you want the family to have a baby. If they argue, interrupt their argument and try again. If they just stand there, try again. Always keep putting them together until they decide to go jump on the bed.

Another thing that helps is to always make sure that you attempt to make a baby every time you start the game up until you have the full number of children that you want. They will usually tell you it's too soon the first time you start the game after the previous baby was born. Putting the parents together right after launching the game will reset that timer and allow you to try again sooner.