I think you need to get your uni involved somehow, is there a college welfare department or anything?
You're really trapped between a rock and a hard place as you can't afford en suite so have had to compromise initially with an agreement that sounded like a fair compromise for all parties, but has rapidly descended into an unfair compromise due to boundary issues, ability to lock doors and enforce those boundaries... Abdul a rapidly deteriorating landlady (?) relationship.
Your uni may be able to help or suggest alternatives as I think you're going to end up very unhappy there if the landlady person keeps up her behaviour and the relationship becomes irretrievable.
I'd also push again on the 'health grounds' aspect, and what might be done with that now you find yourself in this situation (people may be able to be more flexible now if they're able).
I'm sorry you are in this situation.
Btw I have a very newly turned 7 yr old and he wouldn't dream of behaving like this... but I've taught him to respect other people's belongings, including mine and his own. I have a feeling this little chap hasn't had the same teaching! With young children it takes a long time for them to get the hang of certain things, so it's different from saying 'you must x' and they do it, as you'd expect an adult to be able to do. But... this is not your fault and the parent should modify their behaviour so you aren't affected by her child. That's what good parents do! But if she's never bothered to teach her child that he can't have everything he wants by taking it, well, it will take him a lot longer to learn (if he ever does), compared to children who've had years of positive reinforcement from their parents. Sorry, hard not to sound a wee but sanctimonious using myself as an example, so ignore that tinge of sanctimony, it's not meant (!). I think most parents teach these basics to their children as part of daily life even if not consciously. It really is the adults fault in your situation, not the child's. Sounds like the adult has a grabby sense of false entitlement and that's who the kid has as a role model. Which is what you're running up against.
And NO it isn't like waving sweets under the child's nose or eating in front of a starving man etc! And the OP certainly didn't 'ask for it', sly bit of victim blaming there.
Children learn from an extremely early age that there are some things they cannot have and that there is a difference between sharing in their family, and the way you behave to adults outside your family. Eg a toddler may try and grab an iPad from the stranger in the next seat (That was an awkward plane journey I can tell you! Poor man was very nice about it but looked harried around the eyes, and I did everything I could to minimize my toddlers impact on his journey). But by 6yrs old I'd expect no such behaviour... because children learn. Likewise DS doesn't touch my mobile phone, because he knows it's not for him, full stop. It doesn't torture him to see me carrying it around and (gasp), using it in front of him. I know DS would love to play on my phone but he doesn't because it's for grown ups. Likewise all my technology is either completely off limits or has very clear rules about permission and behavior e.g. iPad every Saturday for 20 mins. But no to my laptop at any time etc.
And DS isn't some poor tortured soul because he can't grab anything he wants from adults around him.