Doors and locks. I tried just doors and found a peanut butter jelly sandwich remnant in the tape player.
But as pointed out, by TerryO, store it away or interim system can work too, since they will grow up all too quickly.
I see some logic to that-- a simple BPC hooked to a DVD, TV and 2 non-precious speakers. Preferably with big analog knobs and switches so he can learn. (I put huge orange pieces of shelf paper on the ones he was aloud to touch.) If a PB&J goes into the VCR, let him sit with you while you clean it out... saying "yuck" or whatever you guys use to communicate something bad. If you don't put the blame on him but let him be part of the solution *maybe* he will form a vested interest in "protecting" the equipment that he and Dad take care of. (that's what I did with out PB&J incident, I don't really know if it worked that way inside his little head, but he did move to vehemently protecting the stuff from other kids. Even Mom; if she didn't wash her hands first. Which gave me no end of joy, even if I did have to run out of the room in order to laugh.)
IMO, my kids are worth more than my gear. And my sofa (which we sold and replaced with one from good will. Why? So the kids could climb and jump and pull out the cushions and make forts...) I determined not to have anything precious in the house that would cause a rift between me and the kids. It (mostly) seems to have worked.
I did notice that when I was setting up the post-toddler system my kids were watching carefully-- the care, precision and general fussiness involved fascinated them, and from then on they tried to treat everything in the same manner. This was quite strictly policed by oldest to youngest, and I had to do very little. I was very eager to teach them about the different pieces, set them up in the sweet spot and do some comparative listening. None of that really made sense until we watched a movie (LOTR if I remember) and they declared the new system "awesome, even if it is more scary". High praise from a 3 yr old.
They have almost always treated all of my things with the utmost respect, especially guitars-- which oddly enough they a have always been encouraged to pull down off the wall, plug in and use. The more I encouraged the more careful they were.
This is all anecdotal of course, all kids are different, but I sure hope yours are as fun as mine are-- you are in for one heck of a blast. It's the best.
At this point, I have a 16 year old who only leaves his room so he can plug the macbook into the main LR system, even if he's just browsing YouTube, a daughter is griping because I haven't built a set of speakers for her and "
why can't
I have a tube amp in my room?" (the answer is "not until you foreswear Justin Bieber and other autotune atrocities") My youngest? "I don't really like music unless I make it myself or other people are there making it, that's when I like it" :zoom:

There is hope for this generation.
Are they audiophiles? No, the gear is just a means to an end, the end is the exploration of new music. I'm ok with that.
Having said that, it gets more and more and more fun. No matter what you do with the gear now, the road ahead is a blast. You have NO IDEA what a wealth of incredible tunes my oldest has hooked me up with, it is a
blast
*please don't get the idea that I'm super-dad, or that I think I'm super dad, as the years go on I more and more clearly see the mistakes I made, they break my heart. Then occasionally, I remember something that I *may* have gotten right. Thank God kids are resilient, they need to be with a Dad like me. :sigh:
HTH
Mike