If you have young kids, you need to enjoy them while they're young.
Sure your kid might be 5 years old with decades of life ahead of him, but by the time he's 10 over half the time you will get to spent with him will have passed. By the time he finishes college more than 95% of the time you will get to spend with him will have passed.
The chart below illustrates this.
Your experience may vary, but will follow a similar path. You are everything to your kids until they start school. Then you're a little less-than-everything as they get older, meet friends and get bogged down with schoolwork. Finally, by the time they start working and move out you're the person they hopefully visit once a week for a couple hours.
Invest time in your kids while they're young for the once-in-a-lifetime experience of raising your offspring. Invest time in your kids while they're young to set the foundation for a good adult relationship.
This is why I leave the office at 5pm. This is why I avoid working in the evenings or weekends. This is why I prioritize going home to my kids over drinking with my work buddies.