One of the first things I notices was how slowly a short pair of kid legs can go. My nephew and I were walking to the neighborhood park, and I turned to see my nephew lagging about a house behind me.
I had to re-learn kid pace.
Then there is the issue of picky eating, a facet of childhood that has largely disappeared from our meals. If one of my adult children gets picky, well, too bad: they know where the bread and bologna can be found. With a smaller child you have to find a way to accommodate the limited range of foods the kid will even pinch his nose and eat.
I also ran into that "just-so" mindset of five-year-olds, in this case the way my nephew liked his shoelaces "not too long but not too short." He can tie them, but not in the perfect manner he prefers. We spent 20 minutes one day just trying to get the laces dead-on, and though I kept my cool, I have to admit my patience began to wear thin after about the 20th attempt to attain laced perfection.
After a week with us, my nephew went back home, and both my wife and I were a bit exhausted. I had forgotten how much physical effort goes into taking care of younger children, and though we will miss him until the next visit, my nephew's presence was also a reminder how glad I am that we are finished being full-time caretaking parents.