With children that cry like the two year old it's typically seperation anxiety. Did this child have his mom watch him up until this point or something. Can you help him get a buddy in the day care setting.
As far as the infants go it's typical. However, I don't personally believe babies cry for no reason. Sometimes, if you are able to pick up the baby and you go shhhhh in it's ear and do it as loud as the baby is crying a lot of times that will work. Is their anybody else in your day care that can help out by holding a baby for you?