I have a DD who has curly hair but i wouldnt straighten her hair at this age as she is only 2. I think upper primary school its fine but i wouldnt at this young age.
I was asked by mil if she could straighten it when she was 1 1/2 and i was like NO Way!! But saying that my sil reguarly has striaghtened her dd hair since this age. So i guess everyone is different. Me personally though i wouldnt do it until at least 8-10 years of age for a special occassion.