Please, DON'T TOUCH

I would recommend a combination of what has already been said. First, unplug the power from everything. Second, turn the speakers to face against a wall, plugging any ports, and draping cloth over them. Third, disconnect any turntable and hide it on an upper shelf in a closet. Fourth, drape cloth over remaining components. Fifth, tell the parents that your rig is out of service and in a fragile state and you would appreciate it if no one touches it, urging them to communicate this to the child. Good luck.

this is a hard thing to do! Having 2 boys less than 6 yrs old that are curious kittens, I can empathize w/ your situation. Little boys love buttons to push! :-) The more, the merrier for them!

well, if you have speaker grills, then deploy for sure. They can protect the drivers unless the child does something drastic. You can also mark the speaker position on the carpet w/ tape. Then move the speaker against the wall w/ the drivers pointing into the wall i.e. the child can only play w/ the speaker binding posts. This ain't so bad.
Another thing you can do is, unplug the equipment so that it does not respond to the button push. The little fingers want to push the buttons & see the equipment light up. when it doesn't (it's unplugged) their enthusiasm wanes rapidly & they leave that equipment alone. Also, know that buttons on stereo equipment are designed for many pushes before it actually fails.
3rdly, if you have spare bedsheets, thrown them over the gear (much like you would over furniture when you paint the walls).
FWIW.

Well, I have been in this situation, and what I did was this:

First, be completely honest with the parents, and let them know that you are very concerned.

Second, let them know that you are aware that kids will be kids, but if something gets broken, that as the parents of the child, they are responsible for the child's actions, including any damaged or broken equipment.

Third, and this is the most important advice I am giving, let the parents know the exact cost of the components of your system. (Yes, this might seem like bragging, or it might make them think you're eccentric at best, and nuts at worst, but you will get your point across that if their kid breaks it, they will be out a very large sum of money!)

At this point the parents will be very concerned that they will now be on alert and will be quite aggressive towards watching the child.

FYI, I've done this a couple of times, and one time, (my wife's friends), actually said they changed their minds and ended up staying at a local hotel with their two children. (Didn't break my heart in the least!)

My two cents worth anyway!
Good Luck to you and your equipment!

PS If you have a turntable, make sure to:
A. Have a dust cover,
B. Always have it in place,
and,
C. If possible, put something heavy on top, so they kid can't lift it up!

FYI: At one of my BBQ's, my friend's kids strummed the exposed cantilever of my Benz Micro Glider 2 like a guitar string so they could hear it make funny noises! (My friend (obviously a true friend!) came through with most of the money to replace the cartridge. I got a Koetsu next time, as the cantilever is hidden under the body of the cartridge and it has a stylus guard to boot!!)

Remove all doubt, pack it away & not have to be offending to your friends. It'll sound that much better to you when you fire it up after they vacate. peace of mind

Better yet tell your freinds you will meet them for coffee at a local coffee shop