I would not 'punish' the crying however I would ensure it gets NO attention from anyone at all ... at that age if a child is needing to have a good cry for whatever reason they are taught to go have some 'alone time' on their cot and as soon as they are ready they are more than welcome to return to the group at their own initiative ... so it is not a time out in the traditional sense that I am determining how long they are sitting there it is more teaching them 'self regulation' with the message is it is ok to be SAD that mom and dad are gone however being here is not a choice and it is not ok to ruin everyone elses enjoyment of the program just cause you are feeling sad ~ so here is a place where you can experience your emotions and reflect on your choice is to either be sad here in your alone space or make the best of that fact you are here like it or not and come join your friends for some fun.
I find when they are not getting any 'control' over their behavior getting looks from peers or peers complaining it is too loud and well getting no 'attention' for the crying and so forth that they are much quicker to choose to come to the group and make the most of it where they will get attention and interaction from others.