ok after reading that its your children that you do not want to find your secret garden.. what i would suggest in all fairness would to simply for example get yourself depending on size requirements a 60x60 or 90x90 grow tent, these are smaller than average and have height ranging from 120cm to 160cm. With the aid of a small carbon filter, fan and ducting this should be perfect.
All you need to do is to find somewhere secure to tuck this in your home. We dont want the kids snooping around or to stumble upon this. You could for extra security get yourself a small padlock the type you may use to lock your luggage when you go on holiday. if the kids did find this small unit you could give them some bullshit excuse like its a pop up storage for your tools or something.
i am not sure i like the idea of growing under your house in the open and if anyone can access this area then its kinda a big no no for me.
i am positive that you will find the best way that suits you, there is brilliant ideas always flying around ( AFN ) and everyone is so helpfull you will be sure to get up and running in no time.