There are several things you can do before getting mods.
First, have a sim join a new career and meet coworkers. This will force the game to generate more sims.
Second, have a sim join the daycare profession. This should generate some toddlers and children.
If you want to resort to mods, you can try Twallan's story progression mod for a better progression and with his master controller you can generate children for families. Awesomemod has a great story progression to it, but it's a hefty mod with lots of bells and whistles that you may not want. It does have some commands that will generate sims for you (spawn more overlords is one that will fill the town, babyboom gives every family a toddler and an infant, there are others). I have had good luck with buzzler's birth control mirror but you need to have child-bearing aged sims in relationships for that one to work, so it's too late to use it if your town is too elderly.
