If you want to give them milk, buy kitty milk, it doesn't upset their system.

Also, try to learn cat language. Cats are not dogs, so people assume they don't need training. If you know cat language you can train them to behave nicely.

ETA - I would add to the pp and say *always* keep them in at night time, regardless of vax, age, etc. When kitty gets a bit older and only needs one meal a day, call them in at dusk, feed them fresh food then, and put their dry food away in the morning. Put a minimum of 2 bells (different shapes/sizes) on their collar (they can walk silently with one) & always use a collar with a stretchy elastic insert so they can't strangle themselves going through a tight gap somewhere.