Plat Elite Companion Upgrades go thru 5 days in advance(if avail) unlike Gold. Since you said your plat, you can try calling the elite line and let them know that you have a 11 yr old companion joining up with you in MSP and would like them added to your itin. I dont know if they can/will do so, but if they do, you put the 11 yr old on your itin and then able to be auto upgraded(if avail) 5 days in adv and above the golds/silvers.