I would have bought them less stuff. I was hyperfocused on that stuff when they were small (best clothes, best and latest toys, etc). They ended up being kind of spoiled and not really appreciative of their things. It was also a huge waste of money that I wish I had set aside for other things.
I agree with the person upthread who said better eating habits. I also wish I had pushed sports. My kids are lazy, I wish they wanted to be more active.
I also wish I had worked at least part time. Ive been a SAHM for almost 18 years and while it's been great for the kids, it's been really hard for me