P.R.
I am in the process of starting a blog on this very subject! Unfortunately, it's not up and running, and I don't want to give away too many of my ideas, but here are a couple:
You specifically asked how to still have fun. You have to turn to the simple joys. Pack picnic lunches and find beautiful spots to take them. There are so many gorgeous parks and beaches. Remember that small children don't have a clue how much money you're spending, they just want your time. Buy some bubbles (so cheap!) and spend an hour blowing them in the backyard. See how happy that child is.
I saw someone mention they are doing Netflix and dropped cable. We sort of did that too, but we dropped just the premium (movie) channels and kept Netflix. And while we're on the subject of movies...don't buy videos for your kids, rent them! With Netflix you can keep them forever so your child can watch the favorites over and over again until they get tired of it. Or even go to the library for movies, although the selection is usually hit and miss.
Most communities offer a plethora of free or very low-cost classes and activities like movies in the park. Check your local rec dept or community college/adult ed.
Other than that, my best advice is it's a constant job juggling expenses. No matter how much you're saving, you can probably save more. Once you start, you can usually find more ways by being creative. Well...I can't say too much more but if I get my blog up soon I'll post the link here!