Trying to "Cross-Commission" from ROTC is very difficult. It can be done from one of the academies, but ROTC (since the CG does not have one) is very difficult to do. I have never heard of or met anyone who did it. A FAR better way is to graduate college and apply for OCS, that way your career options are all open. Any of the direct commissioning programs (maritime colleges, direct commission, or other program) limit your career choices dramatically to your college specialty under normal circumstances. IE: Maritime Academy Grads are normally limited to shoreside marine inspection, facilities management, etc. Direct Commission engineering grads are usually limited to shoreside engineering project management or planning.
One can still promote, have a good career, but will never have command opportunities. ROTC Programs are structured to produce Line/Field Officers and count on those grads to fill needed slots in operational units. I have heard there is a "reserve" ROTC Commission that is possible, but you have an eight year obligation, can be called up at any time, and still have an initial AD obligation. Usually the "reserve" is granted for a new officer to immediately attend grad school in a specialty, then upon graduation serve AD obligation.
One idea for your child......do NROTC (I don't think the CG would accept AFROTC or AROTC), Commission, then after their first sea tour try for an "exchange" program where they will serve on a CG unit for two years or so, get to meet the people who can help, then apply to change services.