Sadly your parents will need to reclear security as they are changing terminals from T5 to T3. So they will need to wait for the assistance staff, who are not BA, to arrive at T5 and take them accross to T3. Their flight to MIA will be on AA, so seat selection will follow their policy. They will have a separate AA pnr, which can be obtained from BA. 90 minutes should be enough if all goes well and their first flight is on time, but it doesn't leave any room for delays. Good luck with it.