My boys are 3yrs and 7months. We get slammed by their demands at this stage of their lives. It's hard but do-able.
I negotiate with my girlfriend. I make sure I spend enough time with them (and her alone...I'm not just a father, I'm also her boyfriend and her friend, so I also need to make time for her). I call this gf points. So, I use them so I can go to a user group meeting or a conference on weekends without feeling guilty. I let her go out by herself and I stay home watching the two little guys. It's usually war but we manage to survive. She gets a well-deserved break from us.
I involve them. My girlfriend knows how much I enjoy what I do for a living. So, I ask her to test my projects, or just give me her user point of view feedback on things. Heck, even the 3yr old watches me type as I write code. As far as he knows, it's just numbers and letters but he still pays attention. Sometimes I hold on my lap the 7month old baby. He pays attention to the screen, I'm surprised at this one. This one had a funny side effect, one day I showed my gf baby smash from Scott Hanselman...I get kicked out from my machine quite often because they want to play it (only for a few minutes)
I use whatever chance I get. I suck at getting up early in the morning. So, while my gf cooks breakfast, I hold the baby and read a blog post or something. The 3yr old usually watches tv or entertains himself with his ipod learning letters and numbers or just drawing like any 3 year old. When they nap, I make the best of it.
I suck it up. Sometimes I come home inspired, dying to try something cool but once I get home I am greeted by them...and it's hard to say no to them with their big smile. I figure that if I am really that interested in something, I will make time for it one way or another without sacrificing time with them.
Quality over quantity. This is becoming more important to me. My 3yr old teaching me this. Before he, as my 7month old, required a lot of my attention. Now, all he wants is to show me what he did in school for the day, and/or practice his numbers/letters/shapes etc. Normal kid stuff. Sometimes he just wants my undivided attention for 20-30mins.
I adapt, and not fight it. I can't use the same technique every day. There's been days when I do want to stay late and code but when my 3rd old says: "C'mon papi, let's go to bed pls", it's hard to say no.