I might be doing paid DM work for 8-12 year olds
This statement leads me to so many questions: Are you being paid to babysit and are DMing, or being paid to just DM? Are other adults/parents present? How many kids? Where (FLGS, your home, a birthday party, etc)? All that aside, here's a couple tips.
If at all possible, have another adult present. That adult can be in charge of adulting, while you're in charge of gaming. My spouse and I did this with our kids (and their friends), and tag teaming with an extra adult went a long way to minimize stress.
If you're playing with minis, have a couple extra designated for the kids to fiddle with in front of them. They're going to want to play with the minis on the board, and having extras to act as fidgets will go a long way to minimizing unnecessary touching of the game space.
Plan on extra time for shopping.
Feel free to abuse the basics. You can use the most cliche and overused gaming tropes and the kids will think it's amazing and original. Monsters that you think are boring will be amazing. Make the most of that wonder.
Don't allow any food or drink at the gaming table unless you're okay with your minis getting sticky. Even then, plan on your minis getting sticky.
If possible, I recommend having characters that are ~75% made ahead of time. Character sheets with classes, spells, feats, etc, picked out. But no names, no ability scores, and maybe a couple other choices left blank. Let the kids feel like it's their character by rolling/buying scores, giving names, choosing weapons, and maybe a couple of other things. But don't expect them to actually build something from scratch.
And very much a personal preference of mine: start at level 1.
Good luck!