What phonetic sounds are some of the last sounds for a child to master?
The phonemes /k/ and /t/ may be some of the last sounds a child might master. Some children may not master their differences until as late as 7 years old.
Interestingly when dictation on smart phones first came out people may have noticed that the words they spoke into their phones or devices were typing "k" when a /t/ sound was spoken or the device typed a "t" when the /k/ sound was spoken. Hearing the differences between these two sounds greatly depends on understanding how they are produced when speaking, something technology took time to learn.