How old is she? When I've had tiny kittens they eat little and often, but always tin or packet kitten food. Her little tummy might not yet be ready for biscuit, that can be binding without anything else. Also don't dilute the kitty milk (unless they tell you to), it's the right consistency in the bottle to give her the nourishment she needs. Quick snacks, quick play, lots of sleep, are what kittens need! (Poop and wee may only be once a day, that's normal) but you might have little accidents until she gets used to your tray.