content: It is definitely ripe in the literature of Developmental Psychology, however there is good debate over if it us truly masturbation, as that would mean that they are seeking sexual gratification. My layman's assumption is that it is actually growing pains in the sense that they have rapidly developing nerves in their genitals, which results in rubbing and touching and over-stimulation.  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2994165/