Why I Stopped Hitting My Kids

This title is somewhat misleading, but bear with me for a second. I technically only have one child, but since the end of 2015 I have taken two other kids under my wing.
You see, in my inexplicable ambition of making my life as hard for myself as possible, I decided to move thousands of miles away from my family to take a salary that was so low it would be considered criminal in my home country, to start dating a woman who already had two children. Not long after, she fell pregnant. Although I would never self-diagnose myself, I am told this type of risky behaviour is typically characteristic of someone who has ADHD.
That being said, I replaced copious amounts of alcohol with being a fulltime father, and took on the two other children as best I could. The problem was, they had already been to some extent raised by someone else (the grandparents), and were used to a certain way of life. This involved being spoilt to a point where only when they had crossed a certain line that would directly impact on an elder would they be disciplined, and that discipline would take the form of a smack or two.
Anyone who has ever taken on kids that have been brought up in a particular household will tell you that it is very easy to continue the practices of those who have already been raising them. No actual life coaching is given, no explanation on how to behave, how to act, how to talk - nothing. But when they cross a certain line, they receive a smack.
After a long break whilst these so-called grandparents took the other two on a holiday overseas, the kids are back in my life and I have never seen them happier in all the time that I have known them. Rules, guidelines and bedtimes are enforced, but not with the backing of my hand. They are enforced through an understanding and through a lot of compromise. I work a lot and so does my partner, but when the kids are with us we spoil them with our attention. I have not 100 per cent broken them of their bad habits and certain things they were used to with their grandparents, but we are slowly moving them away from that lifestyle and into a happier, healthier and more rewarding lifestyle. It takes a lot of work, and I still get frustrated. But it's worth the effort.
Any decision on my part to undo all of my hard work and start hitting my kids would be nothing short of selfish. It benefits no one, particularly the kids. I hope others can learn from this experience.