I've been working at home 3 days a week since my youngest was born (the older kid were 2 and 8) and my husband works from home as well. Keeping them away from our equipment has never been an issue and we don't even have a dedicated office space. My husband has a regular old desk in our family room where his computer, monitor, files, desk phone and printer are. I work totally on a lap top now which is on the kitchen table. They just know to not touch our equipment, ever.
I think the secretary desk is a great idea and it's good that you're thinking ahead, but it might not be that big of a deal anyway. Presumably you lock your screen every time you walk away from your computer so the biggest hazards are physical. Just make sure that you properly back up your files frequently and don't allow anyone to bring snacks or drinks anywhere near where your computer is. Keyboards are super-cheap so I don't worry about those too much and monitors aren't a big expense either. In my job, the data on my laptop is many times more valuable than the equipment itself, so just make sure that you have everything secured and backed up in case of an accident at home.