they can't stop you from taking your daughter to see her grandfather.
They can, that is why hospitals have security guards and in many cases police on scene at all times. It is not just to contend with drunks, and as has been pointed out above there are very good reasons in certain cases.
I do sympathise with people wanting to take their children onto wards but the sad fact is that an awful lot of people have no regard for others and would undoubtedly let their children run wild/make a noise/interfere with equipment etc.
And to the OP, you don't need evidence (there is plenty) to realise that the more people entering a ward, the higher the chance of infection. Hope your father-in-law makes a good recovery and yes, it may well be worth having a chat with the ward Sister and once she sees you are a reasonable person she may well allow your daughter to see her Grandad, possibly in a side room if that is practicable. Good luck