An appropriate post-hoc test for Kruskal-Wallis is Dunn (1964) test. You could use Dunn test, look at only the comparisons you are interested in, and use a p-value correction † based on only those comparisons you are interested in.
There are other appropriate post-hoc tests.
The problem with using multiple Mann-Whitney tests is that each will be discarding information from the other groups. Results can be incommensurate or difficult to interpret. (See Schwenk dice.)
Dunn test essentially keeps the ranking of values from the original Kruskal-Wallis and avoids this problem. This is analogous to using pairwise t-tests with pooled variance from all groups. (See, for example, how R conducts pairwise t-tests.)
† In most cases, Bonferroni correction is overly conservative. You might consider Holm, or other methods used natively by R. See the Details section at that link for a list and very brief discussion.