When I flew as a kid, I remember my parents milking the pre-boarding for families with "small children" for quite a while :) After all, it sure was nice to be able to get onboard and settled in as soon as possible!
Getting onboard early to stow your stuff is even more important these days, with airline baggage fees encouraging travelers to carry on as much as possible. But many airlines have dropped pre-boarding for families with small children.
Fortunately for us, Hubby and I both have lifetime Gold elite frequent flier status on American Airlines. (Lifetime Gold is given for 1 million miles earned. Unlike many airlines, American counts bonus miles, credit card miles, etc. towards million miler status, so we were able to earn it pretty easily with just a few years of heavy travel -- pre-kids, of course.) One of the benefits of elite status is that we can always pre-board. Gold is the lowest level of status, so we board after first class and the two higher levels of status, but that still allows us to get onboard before most of the other passengers.
Still, we've found that pre-boarding with really small kids isn't always a benefit. Pre-boarding means 20-30 more minutes stuck with a squirmy child in a small seat inside a metal tube. Not. Fun.
So when Hubby and I travel together, we divide and conquer. He pre-boards with most of our stuff. This allows him to secure overhead bin space and get the car seats installed (if necessary). I wait with the kids in the terminal until the very end of boarding. This allows them to burn off a little more energy prior to the flight!
When I fly alone with the kids, I still try to board towards the end of boarding. I don't have a ton of carry-ons when traveling alone, because I physically can't carry that much! So overhead bin space isn't a consideration. I leave enough time to get the car seat installed, but that's about it.
So, if you're flying with small kids and your airline doesn't allow pre-boarding, it's not the end of the world :) Take the opportunity to let them run off a little more energy before you board!