My daughter burned the palm of her hand with the iron, should I pop the blister and get the puss out?

No. No. Leave it alone. The immune system is doing that on its own. Pus means the infection is being dealt with. If you pop it, you will increase the chance of another infection, bleeding, and scarring. If it doesnt get better on its own, see a doctor.
Blister. Usually, the fluid beneath the blister is serum. I make every effort to leave the blister intact, since it creates a very good biological dresssing. However, if there is pus beneath the blister capsule, then the burn is infected. An infected burn can easily worsen by extending not only in surface area but in depth as well, converting a second degree burn to a more serious third degree burn.