The reason you have two sets of teeth probably comes down to size. A full set of permanent teeth would be too big to fit into a young child’s mouth. So milk teeth act as a bridge until the jaw is large enough to accommodate a full set of permanent teeth. The last teeth that emerge are your wisdom teeth.
Most frogs and toads breathe (and take in moisture) through their skin through a process called cutaneous gas exchange, but they also have lungs with which they breathe. During the time they stay submerged under water or buried in soil (such as during hibernation) they ONLY breathe through their skin.