I try not to give in. Usually the crying comes and goes every few minutes. I try to tune it out and remind myself it is for their own good.
Going during nap will send the child the message that if they cry long/hard enough that you will come and get them and they will not have to nap.
I want the children to learn the nap routine as soon as possible. It is about 1.5 -2 hours max so it isn't unreasonable that they should sleep that long