Our language has voiced and unvoiced consonant sounds. You can feel whether a sound is voiced or unvoiced by putting your hand on your throat and making the sound. Is there a vibration? If yes, then it is voiced. If not, then it is unvoiced. For instance, F is an unvoiced consonant while V is voiced. Our lips and teeth are in essentially the same place but for F we make no sound in our throat and for the V we do. Initially, children favor voiced consonants and will replace unvoiced consonants with similar voiced ones, saying, gat for instance, instead of cat.
We make sounds by placing our tongues in different ways at different places in our mouths. Some are produced by placing the tongue toward the front of the palate, such as the N sound. These sounds are easier for children to produce and they use them to replace sounds, such as the C and G sounds, which are made more toward the back of the mouth.
We also create sounds depending on how we use the air passing through our mouths. When we hold air and release it suddenly, we're producing what are called stops or plosives, such as P, B, K. Other sounds are produced by restricting air flow, for example with V, F, or S. These are called fricatives. Children find it easier to employ stop consonants and will replace fricatives with stops.
No diapers required for this problem. The sounds L and R are classified as liquids in phonology. They pose problems for children that almost everyone would recognize: replacing the L sound with a Y sound and the R with a W sound. Y and W are classified as glides or semivowels.
Other sound patterns give children trouble as they are learning to speak, such as consonants at the ends of words, consonant blends, and unstressed syllables. Again, these problems are normal and developmental. They are overcome in time.
Children are usually adept at creating all of the sounds of our language by the age of five. If a child is still struggling at that point, consulting a speech therapist is a reasonable course of action. However, since children reach full ability at different rates, there is no call for alarm.