Ask yourself 1 question.
How's a judge gonna feel/react if you get busted growing weed with a MINOR (a very young child who is not yours by the way) in the house? It becomes more than just manufacturing dude. Out here on the East Coast that would get you maximum sentencing and at your parole hearing the board will be like "Nope don't think so". Better think this through and use your head/gut.
I don't think it is doable, especially with the anti drug relatives. You can be damn sure she is going to sneak into the bedroom and open the tent sooner or later, and just one random comment of "My mom and stepdad has a garden in their bedroom" and you are going down (if it is illegal in your country). If it isn't illegal, go for it, and screw the rest.
A solution would be to rent a small room somewhere. Like most students live in. Just make sure the security is good!
Not loong ago i moved in with my girlfriend and here 5yr old daughter i was looking to set up a lil 1.2x1.2 tent but i know wot my girls daughters like shes very inquisitive an like all young kids she has a bit of a big mouth i know if this tent springs up outta nowhere an takes up a fair bit of space in our bedroom shes gonna b askiing questions i dont really have no idea wot to tell here on top of that she oftens stays with relatives who r anti-drugs full stop in there eyes if ur smokin weed u might as well b smoking crack i've got a nice set up an wanna put it to use but the daughter poses a big security risk need to come up with a cover to tell here or disguise it in some way so she doesnt wanna keep looking inside it or describe it in any bait way to any1 after all loose lips sink ships..
If you've had a lot of girlfriends, and they are all like that, ya think maybe some of it could be you?