I have an unusual situation.
My wife and I had to take in my disabled mother-in-law and her two boys 5 and 7, because dad wasn't around to pay the rent. It was our house or the streets.
One day my wife and I were enjoying the intimacy of being a childless-couple and boom: insta-kids.
My job made finding a time to train awkward anyway. When I took on these kids, my training took a major blow. When I come home from work, I'm immediately greeted with a crisis that I have to handle right now. Then it's family meal time, and there I am with a full stomach, which usually kills my aspirations to train.
I would say the toughest challenge is following a program. Programs demand routine and the boys are always throwing a wrench into my routine.
But I am blessed with the fact that the boys have a fascination with my "Taco Bells". Little boys DEMAND attention from older men and want to emulate them. Turns out I'm "it". So when I work out, they want to work out (it's actually pretty adorable). It inevitably turns into roughhousing, which is disguised as a workout with lots of crawling, rolling, squatting, pressing, jumping, etc.
Funny thing about kids is that they can be cocky and think they can beat you up if they have to. As a parent, sometimes you have to communicate that won't work. Exercising in front of them is actually a good way to humble them. The little punks gain some respect when they try to pick up 24kg and see me snatch it with ease.