I painted my 8-yr-old daughter's room this weekend. I learned a couple of things that might be helpful to you. First--taping off the room is not a myth but something professional painters actually do that saves time and hassle.
Second--if you find something that looks like painter's tape in your garage, it may not be painter's tape. Don't wait until the room has been painted to find out. It may be electrical tape, in which case, much of the paint you worked hard to apply will come off when you rip off the tape.
Third--don't use "as seen on TV" tools in your painting. They may fall off on your head as you paint your 10-foot ceiling.
Fourth--if the "as seen on TV" tool falls off on your head once, don't assume it won't fall off again. It will. And maybe it will be on your 8-yr-old's head. They don't like having paint washed out of their hair.
Fifth--when using tarps to protect important pieces from paint splatter, include the carpet as one of the surfaces to be covered.
Sixth--IF your child is of the Rain Man sort (don't rearrange or change anything--EVER!), don't paint the room. Regardless of their pleas (even if they request that you paint it Wild Wisteria and Crystal Aqua) keep it the same...the furniture, the knick knacks, the stuffed animals, the play kitchen, the paint--everything!!! Otherwise you will come home from your birthday mani/pedi to a distraught, ungrateful child who can't sleep in their room because it's ALL DIFFERE NT! Even if they recently complained and griped about the Daffodil Yellow you painted before they ever sprouted an opinion, know that they will immediately recant---"I LOVED the yellow! Why did you change it?????"
In addition, I would suggest right up front (as opposed to later after several failed attempts)----moving all furniture away from the wall, transporting a tall ladder to the room, and buying a reliable extension pole so the applicator doesn't fall on your head.